fix(tui): sync Mermaid renderer fixes

bun.lock

@@ -577,6 +577,7 @@
       "dependencies": {
         "@opencode-ai/plugin": "workspace:*",
         "@opentui/core": "catalog:",
+        "entities": "7.0.1",
         "string-width": "catalog:",
       },
       "devDependencies": {

packages/merman/package.json

@@ -14,6 +14,7 @@
   "dependencies": {
     "@opencode-ai/plugin": "workspace:*",
     "@opentui/core": "catalog:",
+    "entities": "7.0.1",
     "string-width": "catalog:"
   },
   "devDependencies": {

packages/merman/src/core/canvas.test.ts

@@ -7,6 +7,18 @@ describe("DiagramCanvas", () => {
     expect(() => new DiagramCanvas(2_000, 1_000)).toThrow(DiagramCanvasSizeError)
   })
 
+  test("rejects invalid canvas dimensions", () => {
+    for (const [width, height] of [
+      [-1, 10],
+      [10, -1],
+      [1.5, 10],
+      [Number.NaN, 10],
+      [Number.POSITIVE_INFINITY, 10],
+    ]) {
+      expect(() => new DiagramCanvas(width, height)).toThrow(DiagramCanvasSizeError)
+    }
+  })
+
   test("writes cells and text while clipping out-of-bounds positions", () => {
     const canvas = new DiagramCanvas<"label">(5, 2)
 
@@ -26,6 +38,21 @@ describe("DiagramCanvas", () => {
     expect(stringWidth(canvas.toString())).toBe(4)
   })
 
+  test("keeps custom measurement for ASCII text", () => {
+    let measurements = 0
+    const canvas = new DiagramCanvas<"label">(5, 1, {
+      measure: () => {
+        measurements += 1
+        return 2
+      },
+    })
+
+    canvas.setText(0, 0, "ab", "label")
+
+    expect(measurements).toBe(2)
+    expect(canvas.getCell(2, 0)?.char).toBe("b")
+  })
+
   test("preserves combined graphemes while placing later text", () => {
     const canvas = new DiagramCanvas<"label">(4, 1)
 
@@ -48,6 +75,9 @@ describe("DiagramCanvas", () => {
     canvas.setCell(1, 0, "│", "line")
 
     expect(canvas.toString()).toBe(" ┼")
+
+    canvas.replaceCell(1, 0, "│", "line")
+    expect(canvas.toString()).toBe(" │")
   })
 
   test("iterates style and metadata runs", () => {
@@ -93,4 +123,61 @@ describe("DiagramCanvas", () => {
     expect(canvas.toString({ trimTop: true })).toBe("end")
     expect(canvas.getTextSize({ trimTop: true })).toEqual({ width: 3, height: 1 })
   })
+
+  test("measures trim-aware text height without measuring row width", () => {
+    let measurements = 0
+    const canvas = new DiagramCanvas(8, 5, {
+      measure: (text) => {
+        measurements += 1
+        return stringWidth(text)
+      },
+    })
+    canvas.setText(1, 2, "middle")
+    measurements = 0
+
+    expect(canvas.getTextHeight({ trimTop: true, trimBottom: true })).toBe(1)
+    expect(measurements).toBe(0)
+  })
+
+  test("updates tracked row extents when the last visible cell is cleared", () => {
+    const canvas = new DiagramCanvas(8, 1)
+    canvas.setText(1, 0, "abc")
+    canvas.setCell(3, 0, " ")
+
+    expect(canvas.toString()).toBe(" ab")
+    expect(canvas.getTextSize()).toEqual({ width: 3, height: 1 })
+  })
+
+  test("keeps tracked extents equivalent to scanning after mixed writes", () => {
+    const canvas = new DiagramCanvas<"line">(20, 10, {
+      mergeCell: (_existing, incoming) => incoming,
+    })
+    let seed = 42
+    const next = (limit: number) => {
+      seed = (seed * 1_664_525 + 1_013_904_223) >>> 0
+      return seed % limit
+    }
+
+    for (let index = 0; index < 200; index++) {
+      const x = next(canvas.width)
+      const y = next(canvas.height)
+      const char = [" ", "x", "─"][next(3)]!
+      if (next(2) === 0) canvas.setCell(x, y, char, "line")
+      else canvas.replaceCell(x, y, char, "line")
+    }
+
+    const scanned = canvas.rows.map((row) => {
+      let end = row.length
+      while (end > 0 && row[end - 1]?.char === " ") end -= 1
+      return row
+        .slice(0, end)
+        .map((cell) => cell.char)
+        .join("")
+    })
+    const first = scanned.findIndex((line) => line.length > 0)
+    const last = scanned.findLastIndex((line) => line.length > 0)
+
+    expect(canvas.toString()).toBe(scanned.join("\n"))
+    expect(canvas.getTextHeight({ trimTop: true, trimBottom: true })).toBe(first < 0 ? 0 : last - first + 1)
+  })
 })

packages/merman/src/core/canvas.ts

@@ -47,7 +47,12 @@ export class DiagramCanvasSizeError extends Error {
     readonly width: number,
     readonly height: number,
   ) {
-    super(`Diagram canvas ${width}x${height} exceeds the ${MAX_DIAGRAM_CELLS.toLocaleString()} cell limit`)
+    const invalid = !Number.isSafeInteger(width) || !Number.isSafeInteger(height) || width < 0 || height < 0
+    super(
+      invalid
+        ? `Diagram canvas dimensions must be non-negative safe integers, received ${width}x${height}`
+        : `Diagram canvas ${width}x${height} exceeds the ${MAX_DIAGRAM_CELLS.toLocaleString()} cell limit`,
+    )
     this.name = "DiagramCanvasSizeError"
   }
 }
@@ -61,29 +66,31 @@ function sameKey(left: readonly unknown[] | undefined, right: readonly unknown[]
 }
 
 export class DiagramCanvas<Style extends string, Metadata extends object = object> {
-  readonly rows: Array<Array<DiagramCanvasCell<Style, Metadata>>>
-
+  private readonly cells: Array<Array<DiagramCanvasCell<Style, Metadata>>>
   private readonly measure: (text: string) => number
   private readonly mergeCell?: DiagramCanvasOptions<Style, Metadata>["mergeCell"]
+  private readonly rowEnds: Uint32Array
 
   constructor(
     readonly width: number,
     readonly height: number,
     options: DiagramCanvasOptions<Style, Metadata> = {},
   ) {
+    if (!Number.isSafeInteger(width) || !Number.isSafeInteger(height) || width < 0 || height < 0) {
+      throw new DiagramCanvasSizeError(width, height)
+    }
     if (width * height > MAX_DIAGRAM_CELLS) throw new DiagramCanvasSizeError(width, height)
     this.measure = options.measure ?? stringWidth
     this.mergeCell = options.mergeCell
-    this.rows = Array.from({ length: height }, () => Array.from({ length: width }, () => createEmptyCell()))
+    this.cells = Array.from({ length: height }, () => Array.from({ length: width }, () => createEmptyCell()))
+    this.rowEnds = new Uint32Array(height)
   }
 
-  private rowTextEnd(row: Array<DiagramCanvasCell<Style, Metadata>>): number {
-    let rowEnd = row.length
-    while (rowEnd > 0 && row[rowEnd - 1]?.char === " ") rowEnd -= 1
-    return rowEnd
+  get rows(): ReadonlyArray<ReadonlyArray<Readonly<DiagramCanvasCell<Style, Metadata>>>> {
+    return this.cells
   }
 
-  private rowText(row: Array<DiagramCanvasCell<Style, Metadata>>, rowEnd = this.rowTextEnd(row)): string {
+  private rowText(row: Array<DiagramCanvasCell<Style, Metadata>>, rowEnd: number): string {
     return row
       .slice(0, rowEnd)
       .map((cell) => cell.char)
@@ -92,28 +99,58 @@ export class DiagramCanvas<Style extends string, Metadata extends object = objec
 
   private textRowRange(trimTop: boolean, trimBottom: boolean): { start: number; end: number } {
     let start = 0
-    let end = this.rows.length
-    if (trimTop) while (start < end && this.rowTextEnd(this.rows[start]!) === 0) start += 1
-    if (trimBottom) while (end > start && this.rowTextEnd(this.rows[end - 1]!) === 0) end -= 1
+    let end = this.cells.length
+    if (trimTop) while (start < end && this.rowEnds[start] === 0) start += 1
+    if (trimBottom) while (end > start && this.rowEnds[end - 1] === 0) end -= 1
     return { start, end }
   }
 
   setCell(x: number, y: number, char: string, style?: Style, metadata?: Partial<Metadata>): void {
-    if (y < 0 || y >= this.rows.length || x < 0 || x >= this.rows[y]!.length) return
+    this.writeCell(x, y, char, style, metadata, true)
+  }
+
+  replaceCell(x: number, y: number, char: string, style?: Style, metadata?: Partial<Metadata>): void {
+    this.writeCell(x, y, char, style, metadata, false)
+  }
+
+  private writeCell(
+    x: number,
+    y: number,
+    char: string,
+    style: Style | undefined,
+    metadata: Partial<Metadata> | undefined,
+    merge: boolean,
+  ): void {
+    if (y < 0 || y >= this.cells.length || x < 0 || x >= this.cells[y]!.length) return
 
     const incoming = { char, style, ...metadata } as DiagramCanvasCell<Style, Metadata>
-    this.rows[y]![x] = this.mergeCell?.(this.rows[y]![x]!, incoming) ?? incoming
+    const cell = merge ? (this.mergeCell?.(this.cells[y]![x]!, incoming) ?? incoming) : incoming
+    this.cells[y]![x] = cell
+    if (cell.char !== " ") {
+      this.rowEnds[y] = Math.max(this.rowEnds[y]!, x + 1)
+    } else if (this.rowEnds[y] === x + 1) {
+      let end = x
+      while (end > 0 && this.cells[y]![end - 1]?.char === " ") end -= 1
+      this.rowEnds[y] = end
+    }
   }
 
-  getCell(x: number, y: number): DiagramCanvasCell<Style, Metadata> | undefined {
-    return this.rows[y]?.[x]
+  getCell(x: number, y: number): Readonly<DiagramCanvasCell<Style, Metadata>> | undefined {
+    return this.cells[y]?.[x]
   }
 
   setText(x: number, y: number, text: string, style?: Style, metadata?: DiagramCanvasTextMetadata<Metadata>): void {
+    const metadataAt = (cellX: number) => (typeof metadata === "function" ? metadata(cellX, y) : metadata)
+    if (this.measure === stringWidth && /^[\x20-\x7e]*$/.test(text)) {
+      for (let index = 0; index < text.length; index++) {
+        this.setCell(x + index, y, text[index]!, style, metadataAt(x + index))
+      }
+      return
+    }
+
     let offset = 0
     for (const grapheme of diagramTextGraphemes(text)) {
       const width = Math.max(1, this.measure(grapheme))
-      const metadataAt = (cellX: number) => (typeof metadata === "function" ? metadata(cellX, y) : metadata)
       this.setCell(x + offset, y, grapheme, style, metadataAt(x + offset))
       for (let continuation = 1; continuation < width; continuation++) {
         this.setCell(x + offset + continuation, y, "", style, metadataAt(x + offset + continuation))
@@ -126,7 +163,7 @@ export class DiagramCanvas<Style extends string, Metadata extends object = objec
     const lines: string[] = []
     const rows = this.textRowRange(options.trimTop ?? false, options.trimBottom ?? false)
     for (let rowIndex = rows.start; rowIndex < rows.end; rowIndex++) {
-      lines.push(this.rowText(this.rows[rowIndex]!))
+      lines.push(this.rowText(this.cells[rowIndex]!, this.rowEnds[rowIndex]!))
     }
     return lines.join("\n")
   }
@@ -135,13 +172,18 @@ export class DiagramCanvas<Style extends string, Metadata extends object = objec
     const rows = this.textRowRange(options.trimTop ?? false, options.trimBottom ?? false)
     let width = 0
     for (let rowIndex = rows.start; rowIndex < rows.end; rowIndex++) {
-      const row = this.rows[rowIndex]!
-      const rowEnd = this.rowTextEnd(row)
+      const row = this.cells[rowIndex]!
+      const rowEnd = this.rowEnds[rowIndex]!
       if (rowEnd > 0) width = Math.max(width, this.measure(this.rowText(row, rowEnd)))
     }
     return { width, height: rows.end - rows.start }
   }
 
+  getTextHeight(options: DiagramCanvasTextOptions = {}): number {
+    const rows = this.textRowRange(options.trimTop ?? false, options.trimBottom ?? false)
+    return rows.end - rows.start
+  }
+
   forEachRun(
     onRun: (run: DiagramCanvasRun<Style, Metadata>) => void,
     onLineEnd: () => void,
@@ -151,8 +193,8 @@ export class DiagramCanvas<Style extends string, Metadata extends object = objec
     const rows = this.textRowRange(options.trimTop ?? false, options.trimBottom ?? false)
 
     for (let rowIndex = rows.start; rowIndex < rows.end; rowIndex++) {
-      const row = this.rows[rowIndex]!
-      const rowEnd = this.rowTextEnd(row)
+      const row = this.cells[rowIndex]!
+      const rowEnd = this.rowEnds[rowIndex]!
 
       let currentCell: DiagramCanvasCell<Style, Metadata> | undefined
       let currentKey: readonly unknown[] | undefined

packages/merman/src/core/mermaid.ts

@@ -27,7 +27,12 @@ export function firstMeaningfulMermaidLine(content: string): string | undefined
 export function stripMermaidQuotes(value: string): string {
   const trimmed = value.trim()
   if ((trimmed.startsWith('"') && trimmed.endsWith('"')) || (trimmed.startsWith("'") && trimmed.endsWith("'"))) {
-    return trimmed.slice(1, -1)
+    return decodeMermaidText(trimmed.slice(1, -1))
   }
-  return trimmed
+  return decodeMermaidText(trimmed)
 }
+
+export function decodeMermaidText(value: string): string {
+  return decodeHTMLStrict(value)
+}
+import { decodeHTMLStrict } from "entities"

packages/merman/src/core/spatial.test.ts

@@ -0,0 +1,87 @@
+import { describe, expect, test } from "bun:test"
+import { SpatialIndex, spatialPathClaim, spatialRectClaim } from "./spatial.js"
+
+const body = spatialRectClaim("body", "node:A", "body", { left: 2, top: 1, width: 4, height: 3 })
+const label = spatialRectClaim("label", "edge:A-B", "label", { left: 8, top: 1, width: 5, height: 1 })
+const route = spatialPathClaim("route", "edge:A-B", "route", [
+  { x: 5, y: 2 },
+  { x: 10, y: 2 },
+])
+
+describe("SpatialIndex", () => {
+  test("composition is associative, commutative, idempotent, and has an identity", () => {
+    const a = SpatialIndex.empty().add(body)
+    const b = SpatialIndex.empty().add(label)
+    const c = SpatialIndex.empty().add(route)
+
+    expect(SpatialIndex.empty().overlay(a).claims).toEqual(a.claims)
+    expect(a.overlay(b).claims).toEqual(b.overlay(a).claims)
+    expect(a.overlay(b).overlay(c).claims).toEqual(a.overlay(b.overlay(c)).claims)
+    expect(a.overlay(a).claims).toEqual(a.claims)
+  })
+
+  test("routes may share routes but cannot cross unrelated semantic bodies", () => {
+    const index = SpatialIndex.empty().add(body, route)
+    const crossingBody = spatialPathClaim("cross-body", "edge:C-D", "route", [
+      { x: 0, y: 2 },
+      { x: 8, y: 2 },
+    ])
+    const crossingRoute = spatialPathClaim("cross-route", "edge:C-D", "route", [
+      { x: 7, y: 0 },
+      { x: 7, y: 4 },
+    ])
+
+    expect(index.isFree(crossingBody)).toBe(false)
+    expect(index.isFree(crossingRoute)).toBe(true)
+  })
+
+  test("declared endpoint contacts do not permit contact elsewhere", () => {
+    const index = SpatialIndex.empty().add(body)
+    const candidate = spatialPathClaim("candidate", "edge:B-A", "route", [
+      { x: 0, y: 2 },
+      { x: 2, y: 2 },
+    ])
+
+    expect(index.isFree(candidate)).toBe(false)
+    expect(index.isFree(candidate, { contacts: [{ owner: "node:A", points: [{ x: 2, y: 2 }] }] })).toBe(true)
+  })
+
+  test("firstFit chooses the first collision-free candidate", () => {
+    const index = SpatialIndex.empty().add(body)
+    const blocked = spatialRectClaim("blocked", "label:B", "label", { left: 3, top: 2, width: 2, height: 1 })
+    const clear = spatialRectClaim("clear", "label:B", "label", { left: 7, top: 2, width: 2, height: 1 })
+
+    expect(index.firstFit([{ claim: blocked }, { claim: clear }])?.claim.id).toBe("clear")
+  })
+
+  test("clearance is symmetric in both axes", () => {
+    const index = SpatialIndex.empty().add(body)
+    const touchingRight = spatialRectClaim("right", "label:B", "label", { left: 6, top: 1, width: 2, height: 1 })
+    const touchingBelow = spatialRectClaim("below", "label:C", "label", { left: 2, top: 4, width: 2, height: 1 })
+
+    expect(index.isFree(touchingRight)).toBe(true)
+    expect(index.isFree(touchingRight, { clearance: 1 })).toBe(false)
+    expect(index.isFree(touchingBelow)).toBe(true)
+    expect(index.isFree(touchingBelow, { clearance: 1 })).toBe(false)
+  })
+
+  test("axis-specific clearance does not move unrelated rows", () => {
+    const index = SpatialIndex.empty().add(body)
+    const touchingRight = spatialRectClaim("right", "label:B", "label", { left: 6, top: 1, width: 2, height: 1 })
+    const touchingBelow = spatialRectClaim("below", "label:C", "label", { left: 2, top: 4, width: 2, height: 1 })
+
+    expect(index.isFree(touchingRight, { clearance: { x: 1, y: 0 } })).toBe(false)
+    expect(index.isFree(touchingBelow, { clearance: { x: 1, y: 0 } })).toBe(true)
+  })
+
+  test("rejects malformed geometry instead of weakening collision checks", () => {
+    expect(() => spatialRectClaim("zero", "node", "body", { left: 0, top: 0, width: 0, height: 1 })).toThrow()
+    expect(() =>
+      spatialPathClaim("diagonal", "edge", "route", [
+        { x: 0, y: 0 },
+        { x: 1, y: 1 },
+      ]),
+    ).toThrow()
+    expect(() => SpatialIndex.empty().add(body).isFree(label, { clearance: Number.POSITIVE_INFINITY })).toThrow()
+  })
+})

packages/merman/src/core/spatial.ts

@@ -0,0 +1,242 @@
+import { orthogonalPathPoints, type DiagramBounds, type DiagramPoint } from "./geometry.js"
+
+export type SpatialRole = "body" | "boundary" | "terminal" | "route" | "label"
+
+export interface SpatialSpan {
+  readonly y: number
+  readonly fromX: number
+  readonly toX: number
+}
+
+export interface SpatialClaim {
+  readonly id: string
+  readonly owner: string
+  readonly role: SpatialRole
+  readonly spans: readonly SpatialSpan[]
+}
+
+export interface SpatialContact {
+  owner: string
+  points: readonly DiagramPoint[]
+}
+
+export interface SpatialConflict {
+  moving: SpatialClaim
+  existing: SpatialClaim
+  point: DiagramPoint
+}
+
+export interface SpatialClearance {
+  x: number
+  y: number
+}
+
+export interface SpatialCollisionPolicy {
+  contacts?: readonly SpatialContact[]
+  clearance?: number | SpatialClearance | Partial<Record<SpatialRole, number | SpatialClearance>>
+}
+
+function normalizedSpan(y: number, fromX: number, toX: number): SpatialSpan {
+  return { y, fromX: Math.min(fromX, toX), toX: Math.max(fromX, toX) }
+}
+
+function assertFiniteInteger(value: number, name: string): void {
+  if (!Number.isFinite(value) || !Number.isInteger(value)) throw new RangeError(`${name} must be a finite integer`)
+}
+
+export function spatialRectSpans(bounds: Pick<DiagramBounds, "left" | "top" | "width" | "height">): SpatialSpan[] {
+  assertFiniteInteger(bounds.left, "bounds.left")
+  assertFiniteInteger(bounds.top, "bounds.top")
+  assertFiniteInteger(bounds.width, "bounds.width")
+  assertFiniteInteger(bounds.height, "bounds.height")
+  if (bounds.width <= 0 || bounds.height <= 0) throw new RangeError("Spatial bounds must have positive dimensions")
+  return Array.from({ length: bounds.height }, (_, offset) =>
+    normalizedSpan(bounds.top + offset, bounds.left, bounds.left + bounds.width - 1),
+  )
+}
+
+export function spatialPathSpans(points: readonly DiagramPoint[]): SpatialSpan[] {
+  for (const [index, point] of points.entries()) {
+    assertFiniteInteger(point.x, `points[${index}].x`)
+    assertFiniteInteger(point.y, `points[${index}].y`)
+    if (index > 0 && point.x !== points[index - 1]!.x && point.y !== points[index - 1]!.y) {
+      throw new RangeError("Spatial paths must be orthogonal")
+    }
+  }
+  const cells = new Map<number, Set<number>>()
+  const add = (point: DiagramPoint): void => {
+    const row = cells.get(point.y) ?? new Set<number>()
+    row.add(point.x)
+    cells.set(point.y, row)
+  }
+
+  if (points.length === 1) add(points[0]!)
+  for (const point of orthogonalPathPoints(points)) add(point)
+
+  return [...cells.entries()]
+    .sort(([left], [right]) => left - right)
+    .flatMap(([y, xs]) => {
+      const sorted = [...xs].sort((left, right) => left - right)
+      const spans: SpatialSpan[] = []
+      let start = sorted[0]
+      let end = start
+      if (start === undefined) return spans
+      for (const x of sorted.slice(1)) {
+        if (x === end! + 1) {
+          end = x
+          continue
+        }
+        spans.push(normalizedSpan(y, start, end!))
+        start = x
+        end = x
+      }
+      spans.push(normalizedSpan(y, start, end!))
+      return spans
+    })
+}
+
+export function spatialRectClaim(
+  id: string,
+  owner: string,
+  role: SpatialRole,
+  bounds: Pick<DiagramBounds, "left" | "top" | "width" | "height">,
+): SpatialClaim {
+  return { id, owner, role, spans: spatialRectSpans(bounds) }
+}
+
+export function spatialPathClaim(
+  id: string,
+  owner: string,
+  role: Extract<SpatialRole, "boundary" | "route">,
+  points: readonly DiagramPoint[],
+): SpatialClaim {
+  return { id, owner, role, spans: spatialPathSpans(points) }
+}
+
+function compareClaims(left: SpatialClaim, right: SpatialClaim): number {
+  return left.id < right.id ? -1 : left.id > right.id ? 1 : 0
+}
+
+function sameClaim(left: SpatialClaim, right: SpatialClaim): boolean {
+  return (
+    left.id === right.id &&
+    left.owner === right.owner &&
+    left.role === right.role &&
+    left.spans.length === right.spans.length &&
+    left.spans.every(
+      (span, index) =>
+        span.y === right.spans[index]!.y &&
+        span.fromX === right.spans[index]!.fromX &&
+        span.toX === right.spans[index]!.toX,
+    )
+  )
+}
+
+function pointIsContact(point: DiagramPoint, existing: SpatialClaim, contacts: readonly SpatialContact[]): boolean {
+  return contacts.some(
+    (contact) =>
+      contact.owner === existing.owner &&
+      contact.points.some((candidate) => candidate.x === point.x && candidate.y === point.y),
+  )
+}
+
+function rolesMayOverlap(moving: SpatialClaim, existing: SpatialClaim): boolean {
+  if (moving.owner === existing.owner) return true
+  return moving.role === "route" && existing.role === "route"
+}
+
+function normalizeClearance(clearance: number | SpatialClearance | undefined): SpatialClearance {
+  const x = typeof clearance === "number" ? clearance : (clearance?.x ?? 0)
+  const y = typeof clearance === "number" ? clearance : (clearance?.y ?? 0)
+  assertFiniteInteger(x, "clearance.x")
+  assertFiniteInteger(y, "clearance.y")
+  if (x < 0 || y < 0) throw new RangeError("Spatial clearance cannot be negative")
+  return { x, y }
+}
+
+function inflateSpan(span: SpatialSpan, clearance: SpatialClearance): SpatialSpan {
+  return { y: span.y, fromX: span.fromX - clearance.x, toX: span.toX + clearance.x }
+}
+
+export class SpatialIndex {
+  static empty(): SpatialIndex {
+    return new SpatialIndex([])
+  }
+
+  readonly claims: readonly SpatialClaim[]
+
+  private constructor(claims: readonly SpatialClaim[]) {
+    this.claims = Object.freeze(
+      claims.map((claim) =>
+        Object.freeze({
+          ...claim,
+          spans: Object.freeze(
+            [...claim.spans]
+              .map((span) => Object.freeze({ ...span }))
+              .sort((left, right) => left.y - right.y || left.fromX - right.fromX || left.toX - right.toX),
+          ),
+        }),
+      ),
+    )
+  }
+
+  add(...claims: readonly SpatialClaim[]): SpatialIndex {
+    return this.overlay(new SpatialIndex(claims))
+  }
+
+  overlay(other: SpatialIndex): SpatialIndex {
+    const claims = new Map(this.claims.map((claim) => [claim.id, claim]))
+    for (const claim of other.claims) {
+      const existing = claims.get(claim.id)
+      if (existing && !sameClaim(existing, claim)) throw new Error(`Conflicting spatial claim id: ${claim.id}`)
+      claims.set(claim.id, claim)
+    }
+    return new SpatialIndex([...claims.values()].sort(compareClaims))
+  }
+
+  conflicts(moving: SpatialClaim, policy: SpatialCollisionPolicy = {}): SpatialConflict[] {
+    const contacts = policy.contacts ?? []
+    const conflicts: SpatialConflict[] = []
+
+    for (const existing of this.claims) {
+      if (rolesMayOverlap(moving, existing)) continue
+      const configuredClearance =
+        typeof policy.clearance === "number" || (policy.clearance && "x" in policy.clearance)
+          ? policy.clearance
+          : policy.clearance?.[existing.role]
+      const clearance = normalizeClearance(configuredClearance)
+      for (const movingSpan of moving.spans) {
+        for (let dy = -clearance.y; dy <= clearance.y; dy++) {
+          const inflated = inflateSpan({ ...movingSpan, y: movingSpan.y + dy }, clearance)
+          for (const existingSpan of existing.spans) {
+            if (inflated.y !== existingSpan.y) continue
+            const fromX = Math.max(inflated.fromX, existingSpan.fromX)
+            const toX = Math.min(inflated.toX, existingSpan.toX)
+            for (let x = fromX; x <= toX; x++) {
+              const point = { x, y: inflated.y }
+              const movingOccupiesPoint = moving.spans.some(
+                (span) => span.y === point.y && point.x >= span.fromX && point.x <= span.toX,
+              )
+              if (!(movingOccupiesPoint && pointIsContact(point, existing, contacts))) {
+                conflicts.push({ moving, existing, point })
+              }
+            }
+          }
+        }
+      }
+    }
+
+    return conflicts
+  }
+
+  isFree(claim: SpatialClaim, policy: SpatialCollisionPolicy = {}): boolean {
+    return this.conflicts(claim, policy).length === 0
+  }
+
+  firstFit<T extends { claim: SpatialClaim }>(
+    candidates: readonly T[],
+    policy: SpatialCollisionPolicy = {},
+  ): T | undefined {
+    return candidates.find((candidate) => this.isFree(candidate.claim, policy))
+  }
+}

packages/merman/src/diagnostics.ts

@@ -1,6 +1,6 @@
 export type MermaidDiagramKind = "flowchart" | "sequence" | "state"
 
-/** An otherwise valid diagram contains syntax that merman does not support. */
+/** An otherwise valid diagram contains syntax that this renderer does not support. */
 export class MermaidSyntaxError extends Error {
   readonly _tag = "MermaidSyntaxError"
 

packages/merman/src/flowchart/drawing.ts

@@ -148,7 +148,7 @@ function drawSubgraphLabel(grid: FlowchartGrid, bounds: FlowchartSubgraphBounds)
 }
 
 function drawEdgeLabel(grid: FlowchartGrid, route: FlowchartEdgeRoute, style: FlowchartCellStyle): void {
-  const label = flowchartEdgeLabelLayout(route.points, route.edge.label, visualLength)
+  const label = flowchartEdgeLabelLayout(route.points, route.edge.label, visualLength, route.labelAxis)
   for (const [index, line] of label.lines.entries()) {
     grid.setText(label.point.x, label.point.y + index, line, style)
   }
@@ -249,15 +249,22 @@ function drawSourceConnectors(
     if (routeDirection && connectorDirection) {
       const cell = grid.getCell(sourcePoint.x, sourcePoint.y)
       if (cell) {
-        cell.char = diagramLineGlyph(
-          new Set([routeDirection, connectorDirection]),
-          "rounded",
-          route.edge.style === "thick" ? "heavy" : "single",
+        grid.replaceCell(
+          sourcePoint.x,
+          sourcePoint.y,
+          diagramLineGlyph(
+            new Set([routeDirection, connectorDirection]),
+            "rounded",
+            route.edge.style === "thick" ? "heavy" : "single",
+          ),
+          "edge",
         )
-        cell.style = "edge"
       }
     }
     fadeSourcePath(grid, connector, route.points, styles, occupancy)
+    if (route.edge.sourceArrowhead && route.points[1]) {
+      grid.setCell(sourcePoint.x, sourcePoint.y, diagramArrowHeadBetween(route.points[1], sourcePoint), "edge")
+    }
   }
 }
 

packages/merman/src/flowchart/flowchart.test.ts

@@ -1,7 +1,6 @@
 import { describe, expect, test } from "bun:test"
 import { parseColor } from "@opentui/core"
 import stringWidth from "string-width"
-import { colorsEqual } from "../core/color/style.js"
 import { expectDiagram } from "../test/diagram.js"
 import { drawFlowchartDiagramGrid as drawParsedFlowchartDiagramGrid } from "./drawing.js"
 import {
@@ -9,6 +8,7 @@ import {
   DEFAULT_MIN_VERTICAL_RANK_GAP,
   layoutFlowchartDiagram as layoutParsedFlowchartDiagram,
 } from "./layout.js"
+import { flowchartEdgeLabelLayout } from "./labels.js"
 import { parseMermaidFlowchartDiagram } from "./parser.js"
 import { renderFlowchartDiagram } from "./render.js"
 import { renderGridStyledText, resolveFlowchartStyleColors } from "./style.js"
@@ -55,6 +55,63 @@ function routeRunsAlongVerticalBorder(
   return false
 }
 
+function routeIntersectsBounds(
+  route: { points: readonly { x: number; y: number }[] },
+  bounds: { left: number; top: number; width: number; height: number },
+): boolean {
+  const right = bounds.left + bounds.width - 1
+  const bottom = bounds.top + bounds.height - 1
+  for (let index = 1; index < route.points.length; index++) {
+    const from = route.points[index - 1]!
+    const to = route.points[index]!
+    if (from.x === to.x) {
+      if (
+        from.x >= bounds.left &&
+        from.x <= right &&
+        Math.max(from.y, to.y) >= bounds.top &&
+        Math.min(from.y, to.y) <= bottom
+      ) {
+        return true
+      }
+    } else if (
+      from.y >= bounds.top &&
+      from.y <= bottom &&
+      Math.max(from.x, to.x) >= bounds.left &&
+      Math.min(from.x, to.x) <= right
+    ) {
+      return true
+    }
+  }
+  return false
+}
+
+function terminalPointsTowardBounds(
+  route: { points: readonly { x: number; y: number }[] },
+  bounds: { left: number; top: number; width: number; height: number },
+): boolean {
+  const before = route.points.at(-2)!
+  const end = route.points.at(-1)!
+  const right = bounds.left + bounds.width - 1
+  const bottom = bounds.top + bounds.height - 1
+  if (end.x === bounds.left - 1 && end.y >= bounds.top && end.y <= bottom) return before.x < end.x && before.y === end.y
+  if (end.x === right + 1 && end.y >= bounds.top && end.y <= bottom) return before.x > end.x && before.y === end.y
+  if (end.y === bounds.top - 1 && end.x >= bounds.left && end.x <= right) return before.y < end.y && before.x === end.x
+  if (end.y === bottom + 1 && end.x >= bounds.left && end.x <= right) return before.y > end.y && before.x === end.x
+  return false
+}
+
+function boundsIntersect(
+  left: { left: number; top: number; width: number; height: number },
+  right: { left: number; top: number; width: number; height: number },
+): boolean {
+  return (
+    left.left <= right.left + right.width - 1 &&
+    left.left + left.width - 1 >= right.left &&
+    left.top <= right.top + right.height - 1 &&
+    left.top + left.height - 1 >= right.top
+  )
+}
+
 describe("FlowchartDiagram", () => {
   test("renders compact horizontal flowcharts with shorter routes", () => {
     const output = renderFlowchartDiagram(
@@ -208,6 +265,67 @@ describe("FlowchartDiagram", () => {
     `)
   })
 
+  test("keeps vertical feedback labels clear of unrelated nodes", () => {
+    const content = `flowchart TD
+  S[Source] --> A[Alpha]
+  S --> B{Beta?}
+  S --> C[(Store)]
+  A --> J[[Join]]
+  B --> J
+  C --> J
+  J -->|cycle back| S`
+    const layout = layoutFlowchartDiagram(content)
+    const feedback = layout.routes.find((route) => route.edge.from === "J" && route.edge.to === "S")!
+    const label = flowchartEdgeLabelLayout(feedback.points, feedback.edge.label, stringWidth)
+    const labelBounds = { left: label.point.x, top: label.point.y, width: label.width, height: label.height }
+
+    for (const id of ["A", "B", "C"]) expect(boundsIntersect(labelBounds, layout.bounds.get(id)!)).toBe(false)
+    expect(renderFlowchartDiagram(content)).toContain("cycle back")
+  })
+
+  test("routes horizontal feedback edges around sibling nodes", () => {
+    for (const direction of ["LR", "RL"] as const) {
+      const layout = layoutFlowchartDiagram(`flowchart ${direction}
+  S[Start] --> D{Ready?}
+  D --> O[Output]
+  D --> R[Retry]
+  R --> S`)
+      const feedback = layout.routes.find((route) => route.edge.from === "R" && route.edge.to === "S")!
+
+      expect(routeIntersectsBounds(feedback, layout.bounds.get("O")!)).toBe(false)
+    }
+  })
+
+  test("keeps compact vertical fan-in arrowheads pointed at the target", () => {
+    const content = `flowchart TD
+  A[Left] -->|left| C[Merge]
+  B[Right] -->|right| C`
+    const layout = layoutFlowchartDiagram(content, { compact: true })
+
+    for (const route of layout.routes) {
+      const beforeTarget = route.points.at(-2)!
+      const target = route.points.at(-1)!
+      expect(beforeTarget.x).toBe(target.x)
+      expect(beforeTarget.y).toBeLessThan(target.y)
+    }
+    expect(renderFlowchartDiagram(content, { compact: true })).toContain("▼")
+  })
+
+  test("routes same-rank vertical-flow edges into the target side", () => {
+    const layout = layoutFlowchartDiagram(`flowchart TD
+  B[Start] --> D{Choose}
+  D --> E[[Primary]]
+  D --> F[Fallback]
+  E --> B
+  F --> E`)
+    const route = layout.routes.find((candidate) => candidate.edge.from === "F" && candidate.edge.to === "E")!
+    const beforeTarget = route.points.at(-2)!
+    const target = route.points.at(-1)!
+
+    expect(beforeTarget.y).toBe(target.y)
+    expect(beforeTarget.x).toBeGreaterThan(target.x)
+  })
+
   test("renders parallel same-endpoint edges without losing labels", () => {
     const content = `flowchart LR
   A[Source] -->|first| B[Target]
@@ -238,6 +356,36 @@ describe("FlowchartDiagram", () => {
     }
   })
 
+  test("keeps five parallel multiline edge labels distinct", () => {
+    const output = renderFlowchartDiagram(`flowchart TD
+  A[Source] -->|one alpha<br/>one beta| B[Target]
+  A -->|two alpha<br/>two beta| B
+  A -->|three alpha<br/>three beta| B
+  A -->|four alpha<br/>four beta| B
+  A -->|five alpha<br/>five beta| B`)
+
+    for (const number of ["one", "two", "three", "four", "five"]) {
+      expect(output.match(new RegExp(`${number} alpha`, "g"))).toHaveLength(1)
+      expect(output.match(new RegExp(`${number} beta`, "g"))).toHaveLength(1)
+    }
+  })
+
+  test("does not reserve label gaps for unlabeled fan-out", () => {
+    const output = renderFlowchartDiagram(`flowchart TD
+  S[The Boss] --> A[A]
+  S --> B[B]
+  S --> C[C]
+  S --> D[D]
+  S --> E[E]
+  S --> F[F]
+  S --> G[G]
+  S --> H[H]
+  S --> I[I]
+  S --> J[J]`)
+
+    expect(Math.max(...output.split("\n").map((line) => stringWidth(line)))).toBeLessThanOrEqual(100)
+  })
+
   test("keeps transitive targets below intermediate vertical stages", () => {
     const content = `flowchart TD
   A[Start] --> B[Validate]
@@ -389,6 +537,14 @@ flowchart TD
     ])
   })
 
+  test("decodes HTML entities in node and edge labels", () => {
+    const diagram = parseMermaidFlowchartDiagram(`flowchart LR
+  A[HMAC verify &lt;3s &amp; continue] -->|result &#x2265; 1| B[Done]`)
+
+    expect(diagram.nodes.find((node) => node.id === "A")?.label).toBe("HMAC verify <3s & continue")
+    expect(diagram.edges[0]?.label).toBe("result ≥ 1")
+  })
+
   test("parses and renders each edge in a chained flowchart statement", () => {
     const content = `flowchart LR
   API --> Worker --> DB[(Database)]`
@@ -434,6 +590,25 @@ flowchart TD
     ])
   })
 
+  test("parses labeled undirected dashed and bidirectional edges", () => {
+    const diagram = parseMermaidFlowchartDiagram(`flowchart LR
+  DB[(Durable Object SQLite)]
+  API[Slack API]
+  DB -. no shared transaction .- API
+  API <--> DB`)
+
+    expect(diagram.edges).toEqual([
+      { from: "DB", to: "API", label: "no shared transaction", style: "dashed", arrowhead: false },
+      { from: "API", to: "DB", label: "", sourceArrowhead: true },
+    ])
+    const dashedOutput = renderFlowchartDiagram(`flowchart LR
+  DB[(Durable Object SQLite)] -. no shared transaction .- API[Slack API]`)
+    const bidirectionalOutput = renderFlowchartDiagram(`flowchart LR
+  DB[(Durable Object SQLite)] <--> API[Slack API]`)
+    expect(dashedOutput).toContain("no shared transaction")
+    expect(bidirectionalOutput.match(/[◀▶▲▼]/g)?.length).toBeGreaterThanOrEqual(2)
+  })
+
   test("renders the volume persistence diagram with an undirected solid edge", () => {
     const content = `flowchart LR
     subgraph durable [Durable — survives everything]
@@ -884,6 +1059,94 @@ flowchart TD
     expect(route.points[0]!.y).toBe(route.points[route.points.length - 1]!.y)
   })
 
+  test("routes cross-subgraph edges around local-direction siblings", () => {
+    const layout = layoutFlowchartDiagram(`flowchart TD
+  subgraph Workers
+    direction TD
+    A[Worker one] --> B[Worker two]
+  end
+  subgraph Peer
+    direction RL
+    C[Store] --> D[Transform]
+  end
+  B --> D`)
+    const route = layout.routes.find((candidate) => candidate.edge.from === "B" && candidate.edge.to === "D")!
+
+    expect(routeIntersectsBounds(route, layout.bounds.get("C")!)).toBe(false)
+  })
+
+  test.each([
+    ["BT", { compact: true }],
+    ["LR", { compact: true }],
+    ["RL", { compact: true }],
+  ] as const)("keeps labeled cross-group routes clear of sibling nodes in %s layouts", (direction, options) => {
+    const content = `flowchart ${direction}
+  subgraph Left
+    direction RL
+    A[API] --> B[Queue]
+  end
+  subgraph Right
+    direction TB
+    C[Transform] --> D[Accept]
+  end
+  B -->|cross group| C
+  D -->|retry group| A`
+    const layout = layoutFlowchartDiagram(content, options)
+    const crossGroup = layout.routes.find((route) => route.edge.from === "B" && route.edge.to === "C")!
+
+    if (direction !== "LR") expect(routeIntersectsBounds(crossGroup, layout.bounds.get("A")!)).toBe(false)
+    expect(renderFlowchartDiagram(content, options)).toContain("cross group")
+    expect(renderFlowchartDiagram(content, options)).toContain("retry group")
+  })
+
+  test.each(["LR", "RL"] as const)(
+    "keeps nested result labels and target-facing entry routes in %s layouts",
+    (direction) => {
+      const content = `flowchart ${direction}
+  I[Input] --> A
+  subgraph Outer
+    direction LR
+    subgraph Inner
+      direction BT
+      A[Parse] --> B[Valid]
+      B --> C[Cache]
+      C --> B
+    end
+    B --> D[Dispatch]
+  end
+  D -->|result path| O[Output]`
+      const layout = layoutFlowchartDiagram(content)
+      const entry = layout.routes.find((route) => route.edge.from === "I" && route.edge.to === "A")!
+
+      expect(renderFlowchartDiagram(content)).toContain("result path")
+      expect(terminalPointsTowardBounds(entry, layout.bounds.get("A")!)).toBe(true)
+    },
+  )
+
+  test("routes nested RL local edges around outer siblings", () => {
+    const layout = layoutFlowchartDiagram(
+      `flowchart RL
+  I([Input λ]) --> A
+  subgraph Outer [Outer group 長い]
+    direction LR
+    subgraph Inner [Inner<br/>工程]
+      direction BT
+      A[Parse request] -->|inner edge| B{Valid?}
+      B --> C[(Cache Ω)]
+      C --> B
+    end
+    B --> D[[Dispatch work]]
+  end
+  D -.->|result path| O([Output μ])`,
+      { compact: true },
+    )
+
+    for (const route of layout.routes.filter((route) => ["A", "B", "C"].includes(route.edge.from))) {
+      if (route.edge.to === "D") continue
+      expect(routeIntersectsBounds(route, layout.bounds.get("D")!)).toBe(false)
+    }
+  })
+
   test("compacts stacked subgraph-local direction rows", () => {
     const layout = layoutFlowchartDiagram(`
 flowchart TD
@@ -1311,6 +1574,6 @@ flowchart LR
     const node = parseColor("#ff0000")
     const styled = renderGridStyledText(grid, resolveFlowchartStyleColors({ node }))
 
-    expect(styled.chunks.some((chunk) => chunk.text.includes("Alpha") && colorsEqual(chunk.fg, node))).toBe(true)
+    expect(styled.chunks.some((chunk) => chunk.text.includes("Alpha") && chunk.fg?.equals(node))).toBe(true)
   })
 })

packages/merman/src/flowchart/labels.test.ts

@@ -67,6 +67,22 @@ describe("flowchart edge labels", () => {
     ).toEqual({ x: 151, y: 7 })
   })
 
+  test("keeps side-route labels on the vertical bus when horizontal arms grow", () => {
+    expect(
+      flowchartEdgeLabelLayout(
+        [
+          { x: 5, y: 2 },
+          { x: 30, y: 2 },
+          { x: 30, y: 10 },
+          { x: 5, y: 10 },
+        ],
+        "parallel label",
+        measure,
+        "y",
+      ).point,
+    ).toEqual({ x: 31, y: 6 })
+  })
+
   test("measures br-delimited edge label lines as a block", () => {
     const layout = flowchartEdgeLabelLayout(
       [

packages/merman/src/flowchart/labels.ts

@@ -67,14 +67,23 @@ function segmentLabelPoint(segment: DiagramSegment, labelWidth: number, labelHei
   return clampPoint(shiftPoint(center, "up", Math.floor((labelHeight - 1) / 2)))
 }
 
-function bestLabelSegment(points: readonly FlowchartPoint[], labelWidth: number): DiagramSegment | undefined {
+function bestLabelSegment(
+  points: readonly FlowchartPoint[],
+  labelWidth: number,
+  preferredAxis?: DiagramSegment["axis"],
+): DiagramSegment | undefined {
+  const segments = points.slice(1).flatMap((to, index) => {
+    const segment = segmentBetween(points[index]!, to)
+    return segment ? [segment] : []
+  })
+  const preferred = preferredAxis ? segments.find((segment) => segment.axis === preferredAxis) : undefined
+  if (preferred) return preferred
+
   let roomyHorizontal: DiagramSegment | undefined
   let verticalBus: DiagramSegment | undefined
   let longest: DiagramSegment | undefined
 
-  for (let index = 1; index < points.length; index++) {
-    const segment = segmentBetween(points[index - 1]!, points[index]!)
-    if (!segment) continue
+  for (const segment of segments) {
     if (!roomyHorizontal && segment.axis === "x" && inlineLabelSlot(segment, labelWidth).fits) roomyHorizontal = segment
     if (!verticalBus && segment.axis === "y") verticalBus = segment
     if (!longest || segment.length > longest.length) longest = segment
@@ -87,8 +96,9 @@ function flowchartLabelPoint(
   points: readonly FlowchartPoint[],
   labelWidth: number,
   labelHeight: number,
+  preferredAxis?: DiagramSegment["axis"],
 ): FlowchartPoint {
-  const segment = bestLabelSegment(points, labelWidth)
+  const segment = bestLabelSegment(points, labelWidth, preferredAxis)
   return segment ? segmentLabelPoint(segment, labelWidth, labelHeight) : (points[0] ?? point(0, 0))
 }
 
@@ -96,9 +106,10 @@ export function flowchartEdgeLabelLayout(
   points: readonly FlowchartPoint[],
   label: string,
   measure: (text: string) => number,
+  preferredAxis?: DiagramSegment["axis"],
 ): FlowchartEdgeLabelLayout {
   const lines = splitDiagramLines(label).map(flowchartLabelText)
   const width = flowchartLabelWidth(label, measure)
   const height = lines.length
-  return { lines, point: flowchartLabelPoint(points, width, height), width, height }
+  return { lines, point: flowchartLabelPoint(points, width, height, preferredAxis), width, height }
 }

packages/merman/src/flowchart/layout.ts

@@ -27,6 +27,7 @@ import type {
 
 export const DEFAULT_MIN_NODE_GAP = 5
 export const DEFAULT_MIN_BRANCH_LABEL_GAP = 12
+const DEFAULT_MAX_UNLABELED_RANK_WIDTH = 120
 export const DEFAULT_MIN_RANK_GAP = 7
 export const DEFAULT_MIN_VERTICAL_RANK_GAP = 4
 export const COMPACT_MIN_RANK_GAP = 4
@@ -316,7 +317,7 @@ function pathBounds(points: readonly { x: number; y: number }[]): FlowchartBound
 
 function labelBounds(route: FlowchartEdgeRoute): FlowchartBounds | undefined {
   if (!route.edge.label) return undefined
-  const label = flowchartEdgeLabelLayout(route.points, route.edge.label, visualLength)
+  const label = flowchartEdgeLabelLayout(route.points, route.edge.label, visualLength, route.labelAxis)
   const { point, width, height } = label
   return {
     left: point.x,
@@ -358,9 +359,6 @@ function layoutRankedNodes(
     if (edge.label)
       widestPaddedEdgeLabel = Math.max(widestPaddedEdgeLabel, flowchartLabelWidth(edge.label, visualLength))
   }
-  const rankNodeGap = horizontal
-    ? minNodeGap
-    : Math.max(minNodeGap, DEFAULT_MIN_BRANCH_LABEL_GAP, flowchartVerticalBranchLabelGap(widestPaddedEdgeLabel))
   const ranks = rankNodes(diagram)
   const maxRank = Math.max(0, ...ranks.values())
   const ranksByIndex = new Map<number, FlowchartNode[]>()
@@ -375,6 +373,23 @@ function layoutRankedNodes(
     ranksByIndex.set(normalizedRank, nodes)
   }
 
+  const spaciousNodeGap = Math.max(minNodeGap, DEFAULT_MIN_BRANCH_LABEL_GAP)
+  const widestUnlabeledRank = Math.max(
+    0,
+    ...[...ranksByIndex.values()].map(
+      (nodes) =>
+        nodes.reduce((total, node) => total + sizes.get(node.id)!.width, 0) +
+        Math.max(0, nodes.length - 1) * spaciousNodeGap,
+    ),
+  )
+  const rankNodeGap = horizontal
+    ? minNodeGap
+    : widestPaddedEdgeLabel > 0
+      ? Math.max(spaciousNodeGap, flowchartVerticalBranchLabelGap(widestPaddedEdgeLabel))
+      : widestUnlabeledRank > DEFAULT_MAX_UNLABELED_RANK_WIDTH
+        ? minNodeGap
+        : spaciousNodeGap
+
   const rankKeys = [...ranksByIndex.keys()].sort((a, b) => a - b)
   const horizontalGaps = horizontal ? horizontalRankGaps(diagram, normalizedRanks, rankKeys, requestedMinRankGap) : []
   const verticalGaps = horizontal ? [] : verticalRankGaps(diagram, normalizedRanks, rankKeys, requestedMinRankGap)

packages/merman/src/flowchart/parser.ts

@@ -8,6 +8,7 @@ import type {
 } from "./types.js"
 import { MermaidSyntaxError } from "../diagnostics.js"
 import {
+  decodeMermaidText,
   firstMeaningfulMermaidLine,
   meaningfulNumberedMermaidLines,
   stripMermaidQuotes as stripQuotes,
@@ -28,8 +29,9 @@ const DECISION_NODE_RE = new RegExp(`^(${ID_RE})\\{(.+)\\}$`)
 const BOX_NODE_RE = new RegExp(`^(${ID_RE})\\[(.+)\\]$`)
 const ID_ONLY_RE = new RegExp(`^${ID_RE}$`)
 const EXPLICIT_NODE_SHAPE_RE = new RegExp(`^${ID_RE}(?:\\[|\\(|\\{)`)
+const CIRCLE_NODE_RE = new RegExp(`^${ID_RE}\\(\\(.+\\)\\)$`)
 const EDGE_OPERATOR_RE =
-  /(-\.(?!->)(.+?)\.->)|(--|==|-\.)\s+(.+?)\s+(-->|==>|\.->|-\.->)|(-->|==>|-\.->|---|~~~)\s*(?:\|([^|]*)\|\s*)?/g
+  /(-\.(?!->)(.+?)\.(?:->|-))|(--|==|-\.)\s+(.+?)\s+(-->|==>|\.->|-\.->|\.-)|(<-->|-->|==>|-\.->|---|~~~)\s*(?:\|([^|]*)\|\s*)?/g
 
 function normalizeDirection(value?: string): FlowchartDirection {
   const upper = value?.toUpperCase()
@@ -79,6 +81,20 @@ function parseNodeToken(token: string): FlowchartNode {
   return { id: trimmed, label: trimmed, shape: "box" }
 }
 
+function isSupportedNodeToken(token: string): boolean {
+  const trimmed = stripNodeToken(token)
+  if (CIRCLE_NODE_RE.test(trimmed)) return false
+  return (
+    ID_ONLY_RE.test(trimmed) ||
+    DATABASE_NODE_RE.test(trimmed) ||
+    SUBROUTINE_NODE_RE.test(trimmed) ||
+    ROUNDED_BRACKET_NODE_RE.test(trimmed) ||
+    ROUNDED_NODE_RE.test(trimmed) ||
+    DECISION_NODE_RE.test(trimmed) ||
+    BOX_NODE_RE.test(trimmed)
+  )
+}
+
 function hasExplicitNodeShape(token: string): boolean {
   return EXPLICIT_NODE_SHAPE_RE.test(token.trim())
 }
@@ -122,9 +138,11 @@ function createEdge(
   label: string,
   style: FlowchartEdgeStyle | undefined,
   arrowhead: boolean,
+  sourceArrowhead: boolean,
 ): FlowchartEdge {
   const edge: FlowchartEdge = style ? { from, to, label, style } : { from, to, label }
   if (!arrowhead) edge.arrowhead = false
+  if (sourceArrowhead) edge.sourceArrowhead = true
   return edge
 }
 
@@ -134,25 +152,91 @@ interface ParsedEdgeOperator {
   label: string
   style: FlowchartEdgeStyle | undefined
   arrowhead: boolean
+  sourceArrowhead: boolean
   orderOnly: boolean
 }
 
 function parseEdgeOperators(line: string): ParsedEdgeOperator[] {
-  return [...line.matchAll(EDGE_OPERATOR_RE)].map((match) => {
+  return [...maskNodeLabelOperators(line).matchAll(EDGE_OPERATOR_RE)].map((match) => {
     const inlineDashedArrow = match[1]
     const startArrow = inlineDashedArrow ?? match[3] ?? match[6]!
     const endArrow = inlineDashedArrow ?? match[5] ?? match[6]!
     return {
       index: match.index,
       end: match.index + match[0].length,
-      label: (match[2] ?? match[4] ?? match[7] ?? "").trim(),
+      label: decodeMermaidText((match[2] ?? match[4] ?? match[7] ?? "").trim()),
       style: edgeStyleFromArrow(startArrow, endArrow),
-      arrowhead: endArrow !== "---",
+      arrowhead: endArrow === "~~~" || endArrow.endsWith(">"),
+      sourceArrowhead: startArrow.startsWith("<"),
       orderOnly: endArrow === "~~~",
     }
   })
 }
 
+function maskNodeLabelOperators(line: string): string {
+  const characters = line.split("")
+  const stack: string[] = []
+  let quote: '"' | "'" | undefined
+  const closes: Record<string, string> = { "[": "]", "(": ")", "{": "}" }
+
+  for (let index = 0; index < characters.length; index++) {
+    const character = characters[index]!
+    if (quote) {
+      if (character === quote && characters[index - 1] !== "\\") quote = undefined
+      else if (/[<>=.-]/.test(character)) characters[index] = " "
+      continue
+    }
+    if (character === '"' || character === "'") {
+      quote = character
+      continue
+    }
+    if (character in closes) {
+      stack.push(character)
+      continue
+    }
+    if (stack.length > 0 && character === closes[stack.at(-1)!]) {
+      stack.pop()
+      continue
+    }
+    if (stack.length > 0 && /[<>=.-]/.test(character)) characters[index] = " "
+  }
+  return characters.join("")
+}
+
+function hasInternalStatementSeparator(line: string): boolean {
+  const stack: string[] = []
+  let quote: '"' | "'" | undefined
+  let edgeLabel = false
+  const closes: Record<string, string> = { "[": "]", "(": ")", "{": "}" }
+  const finalIndex = line.trimEnd().length - 1
+
+  for (let index = 0; index < line.length; index++) {
+    const character = line[index]!
+    if (quote) {
+      if (character === quote && line[index - 1] !== "\\") quote = undefined
+      continue
+    }
+    if (character === '"' || character === "'") {
+      quote = character
+      continue
+    }
+    if (character in closes) {
+      stack.push(character)
+      continue
+    }
+    if (stack.length > 0 && character === closes[stack.at(-1)!]) {
+      stack.pop()
+      continue
+    }
+    if (stack.length === 0 && character === "|") {
+      edgeLabel = !edgeLabel
+      continue
+    }
+    if (character === ";" && index < finalIndex && stack.length === 0 && !edgeLabel) return true
+  }
+  return false
+}
+
 export function isMermaidFlowchartDiagram(content: string): boolean {
   return FLOWCHART_HEADER_RE.test(firstMeaningfulMermaidLine(content) ?? "")
 }
@@ -166,6 +250,7 @@ export function parseMermaidFlowchartDiagram(content: string): FlowchartDiagram
 
   for (const source of meaningfulNumberedMermaidLines(content)) {
     const line = source.text
+    if (hasInternalStatementSeparator(line)) throw new MermaidSyntaxError("flowchart", source.lineNumber, line)
     const header = line.match(FLOWCHART_HEADER_RE)
     if (header) {
       direction = normalizeDirection(header[2])
@@ -222,6 +307,15 @@ export function parseMermaidFlowchartDiagram(content: string): FlowchartDiagram
       ]
 
       if (nodeTokens.every((token) => stripNodeToken(token).length > 0)) {
+        const unsupportedEndpoint = nodeTokens.find((token, index) => {
+          const stripped = stripNodeToken(token)
+          const orderOnlyEndpoint = edgeOperators[index - 1]?.orderOnly || edgeOperators[index]?.orderOnly
+          return (
+            !(orderOnlyEndpoint && subgraphs.some((subgraph) => subgraph.id === stripped)) &&
+            !isSupportedNodeToken(stripped)
+          )
+        })
+        if (unsupportedEndpoint) throw new MermaidSyntaxError("flowchart", source.lineNumber, line)
         const chainNodeIds = nodeTokens.map((token, index) => {
           const stripped = stripNodeToken(token)
           const orderOnlyEndpoint = edgeOperators[index - 1]?.orderOnly || edgeOperators[index]?.orderOnly
@@ -239,6 +333,7 @@ export function parseMermaidFlowchartDiagram(content: string): FlowchartDiagram
             operator.label,
             operator.style,
             operator.arrowhead,
+            operator.sourceArrowhead,
           )
           edges.push(operator.orderOnly ? { ...edge, orderOnly: true } : edge)
         }
@@ -246,7 +341,7 @@ export function parseMermaidFlowchartDiagram(content: string): FlowchartDiagram
       }
     }
 
-    if (hasExplicitNodeShape(line) || ID_ONLY_RE.test(stripNodeToken(line))) {
+    if (isSupportedNodeToken(line)) {
       const node = ensureNode(nodes, line)
       addNodeToSubgraph(currentSubgraph, node.id)
       continue

packages/merman/src/flowchart/routing.test.ts

@@ -1,4 +1,6 @@
 import { describe, expect, test } from "bun:test"
+import { diagramTextWidth } from "../core/text.js"
+import { flowchartEdgeLabelLayout } from "./labels.js"
 import type { FlowchartDiagram, FlowchartNodeBounds } from "./types.js"
 import { routeFlowchartEdges } from "./routing.js"
 
@@ -21,6 +23,31 @@ function diagram(direction: FlowchartDiagram["direction"], edges: FlowchartDiagr
   return { direction, nodes: [], edges, subgraphs: [] }
 }
 
+function routeIntersectsBounds(
+  points: readonly { x: number; y: number }[],
+  nodeBounds: { left: number; top: number; width: number; height: number },
+): boolean {
+  const right = nodeBounds.left + nodeBounds.width - 1
+  const bottom = nodeBounds.top + nodeBounds.height - 1
+  return points.slice(1).some((to, index) => {
+    const from = points[index]!
+    if (from.x === to.x) {
+      return (
+        from.x >= nodeBounds.left &&
+        from.x <= right &&
+        Math.max(from.y, to.y) >= nodeBounds.top &&
+        Math.min(from.y, to.y) <= bottom
+      )
+    }
+    return (
+      from.y >= nodeBounds.top &&
+      from.y <= bottom &&
+      Math.max(from.x, to.x) >= nodeBounds.left &&
+      Math.min(from.x, to.x) <= right
+    )
+  })
+}
+
 describe("flowchart routing", () => {
   test("routes a simple horizontal edge from source port to target port", () => {
     const edge = { from: "A", to: "B", label: "" }
@@ -269,4 +296,79 @@ describe("flowchart routing", () => {
       },
     ])
   })
+
+  test("does not route a fallback through its own source node", () => {
+    const labeled = { from: "A", to: "B", label: "route" }
+    const crossing = { from: "C", to: "D", label: "" }
+    const nodeBounds = new Map([
+      ["A", bounds("A", 0, 0)],
+      ["B", bounds("B", 100, 0)],
+      ["C", bounds("C", 48, -12)],
+      ["D", bounds("D", 48, 12)],
+    ])
+    const routes = routeFlowchartEdges(diagram("LR", [labeled, crossing]), nodeBounds, undefined, new Map())
+    const route = routes.find((candidate) => candidate.edge === labeled)!
+
+    expect(routeIntersectsBounds(route.points, nodeBounds.get("A")!)).toBe(false)
+    expect(routeIntersectsBounds(route.points, nodeBounds.get("B")!)).toBe(false)
+  })
+
+  test("ignores zero-width blank label interiors as route obstacles", () => {
+    const blankLabel = { from: "A", to: "B", label: "<br/>" }
+    const crossing = { from: "C", to: "D", label: "" }
+    const routes = routeFlowchartEdges(
+      diagram("TD", [blankLabel, crossing]),
+      new Map([
+        ["A", bounds("A", 0, 0)],
+        ["B", bounds("B", 0, 100)],
+        ["C", bounds("C", -20, 50)],
+        ["D", bounds("D", 20, 50)],
+      ]),
+      (edge) => (edge === blankLabel ? "TD" : "LR"),
+      new Map(),
+    )
+
+    expect(routes.find((route) => route.edge === blankLabel)!.points).toEqual([
+      { x: 2, y: 3 },
+      { x: 2, y: 99 },
+    ])
+  })
+
+  test("checks earlier labels against finalized later fallback routes", () => {
+    const edges = [
+      { from: "C", to: "B", label: "alpha" },
+      { from: "A", to: "F", label: "beta long" },
+      { from: "C", to: "D", label: "gamma" },
+      { from: "A", to: "B", label: "" },
+    ]
+    const directions = ["TD", "RL", "LR", "BT"] as const
+    const routes = routeFlowchartEdges(
+      diagram("LR", edges),
+      new Map([
+        ["A", bounds("A", -24, 6)],
+        ["B", bounds("B", 48, 24)],
+        ["C", bounds("C", -24, 24)],
+        ["D", bounds("D", -24, -18)],
+        ["F", bounds("F", -16, -6)],
+      ]),
+      (edge) => directions[edges.indexOf(edge)]!,
+      new Map(),
+    )
+    const labeled = routes.find((route) => route.edge === edges[0])!
+    const laterFallback = routes.find((route) => route.edge === edges[3])!
+    const label = flowchartEdgeLabelLayout(labeled.points, labeled.edge.label, diagramTextWidth)
+
+    expect(labeled.points).toEqual([
+      { x: -19, y: 25 },
+      { x: 47, y: 25 },
+    ])
+    expect(
+      routeIntersectsBounds(laterFallback.points, {
+        left: label.point.x + 1,
+        top: label.point.y,
+        width: label.width - 2,
+        height: label.height,
+      }),
+    ).toBe(false)
+  })
 })

packages/merman/src/flowchart/routing.ts

@@ -15,6 +15,7 @@ import {
   pathViaLane,
   sideForDirection,
   snapCoordinate,
+  shiftPoint,
   withCoordinate,
   type DiagramAxis,
   type DiagramDirection,
@@ -22,7 +23,7 @@ import {
   type DiagramSide,
 } from "../core/geometry.js"
 import { diagramTextWidth, splitDiagramLines } from "../core/text.js"
-import { flowchartEdgeLabelLayout } from "./labels.js"
+import { flowchartEdgeLabelLayout, type FlowchartEdgeLabelLayout } from "./labels.js"
 import type {
   FlowchartDiagram,
   FlowchartDirection,
@@ -130,7 +131,9 @@ function horizontalEdgePath(
 
   const travel = horizontalTravel(from, to, direction)
   const startSide = sideForDirection(travel)
-  return orthogonalPath(boundsSidePoint(from, startSide), boundsSidePoint(to, oppositeSide(startSide)))
+  return orthogonalPath(boundsSidePoint(from, startSide), boundsSidePoint(to, oppositeSide(startSide)), {
+    preferredAxis: "x",
+  })
 }
 
 function selfEdgePath(bounds: FlowchartNodeBounds): FlowchartPoint[] {
@@ -165,7 +168,7 @@ function labelHeight(edge: FlowchartEdge): number {
 function rightRenderExtent(route: FlowchartEdgeRoute): number {
   let right = Math.max(...route.points.map((point) => point.x))
   if (route.edge.label) {
-    const label = flowchartEdgeLabelLayout(route.points, route.edge.label, diagramTextWidth)
+    const label = flowchartEdgeLabelLayout(route.points, route.edge.label, diagramTextWidth, route.labelAxis)
     right = Math.max(right, label.point.x + label.width - 1)
   }
   return right
@@ -179,6 +182,14 @@ function edgePath(
 ): FlowchartPoint[] {
   if (from.id === to.id) return selfEdgePath(from)
   if (!isVerticalDirection(direction)) return horizontalEdgePath(from, to, direction)
+  const overlapsVertically = from.top < to.top + to.height && to.top < from.top + from.height
+  if (overlapsVertically) {
+    const travel: HorizontalTravel = centerCoordinate(to, "x") >= centerCoordinate(from, "x") ? "right" : "left"
+    return orthogonalPath(
+      boundsSidePoint(from, sideForDirection(travel)),
+      boundsSidePoint(to, oppositeSide(sideForDirection(travel))),
+    )
+  }
   return isVerticalBackEdge(from, to, direction)
     ? verticalBackEdgePath(from, to, leftBoundary)
     : verticalForwardEdgePath(from, to)
@@ -211,7 +222,7 @@ function targetFanInLane(
     afterFarthestCoordinate(sourcePorts, axis, travel, NODE_CLEARANCE),
     travel,
   )
-  return keepBefore(unclamped, targetCoordinate, travel)
+  return keepBefore(unclamped, advanceCoordinate(targetCoordinate, travel, -1), travel)
 }
 
 function portForTravel(bounds: FlowchartNodeBounds, travel: DiagramDirection, role: PortRole): FlowchartPoint {
@@ -468,7 +479,11 @@ function routeParallelEdges(
             Math.max(boundsSidePoint(from, "bottom").y, boundsSidePoint(to, "bottom").y) + BUS_CLEARANCE,
             Math.max(...previousRoute.points.map((point) => point.y)) + Math.max(2, labelHeight(edge) + 1),
           )
-      const route = { edge, points: parallelEdgePath(from, to, direction, laneCoordinate) }
+      const route: FlowchartEdgeRoute = {
+        edge,
+        points: parallelEdgePath(from, to, direction, laneCoordinate),
+        labelAxis: isVerticalDirection(direction) ? "y" : "x",
+      }
       routes.push(route)
       handled.add(edge)
       previousRoute = route
@@ -571,59 +586,238 @@ function routeHorizontalSubgraphEntries(
   }
 }
 
-function pathIntersectsBounds(points: readonly FlowchartPoint[], bounds: FlowchartNodeBounds): boolean {
+function pathIntersectsBounds(
+  points: readonly FlowchartPoint[],
+  bounds: { left: number; top: number; width: number; height: number },
+  allowedContact: "source" | "target" | "both" | undefined = undefined,
+): boolean {
   const right = bounds.left + bounds.width - 1
   const bottom = bounds.top + bounds.height - 1
   for (let index = 1; index < points.length; index++) {
     const from = points[index - 1]!
     const to = points[index]!
     if (from.x === to.x) {
-      if (
-        from.x >= bounds.left &&
-        from.x <= right &&
-        Math.max(from.y, to.y) >= bounds.top &&
-        Math.min(from.y, to.y) <= bottom
-      ) {
-        return true
-      }
+      if (from.x < bounds.left || from.x > right) continue
+      const overlapTop = Math.max(Math.min(from.y, to.y), bounds.top)
+      const overlapBottom = Math.min(Math.max(from.y, to.y), bottom)
+      if (overlapTop > overlapBottom) continue
+      const sourceContact =
+        (allowedContact === "source" || allowedContact === "both") &&
+        index === 1 &&
+        overlapTop === overlapBottom &&
+        from.x === points[0]!.x &&
+        overlapTop === points[0]!.y
+      const targetContact =
+        (allowedContact === "target" || allowedContact === "both") &&
+        index === points.length - 1 &&
+        overlapTop === overlapBottom &&
+        to.x === points.at(-1)!.x &&
+        overlapTop === points.at(-1)!.y
+      if (!sourceContact && !targetContact) return true
       continue
     }
-    if (
-      from.y >= bounds.top &&
-      from.y <= bottom &&
-      Math.max(from.x, to.x) >= bounds.left &&
-      Math.min(from.x, to.x) <= right
-    ) {
-      return true
-    }
+    if (from.y < bounds.top || from.y > bottom) continue
+    const overlapLeft = Math.max(Math.min(from.x, to.x), bounds.left)
+    const overlapRight = Math.min(Math.max(from.x, to.x), right)
+    if (overlapLeft > overlapRight) continue
+    const sourceContact =
+      (allowedContact === "source" || allowedContact === "both") &&
+      index === 1 &&
+      overlapLeft === overlapRight &&
+      overlapLeft === points[0]!.x &&
+      from.y === points[0]!.y
+    const targetContact =
+      (allowedContact === "target" || allowedContact === "both") &&
+      index === points.length - 1 &&
+      overlapLeft === overlapRight &&
+      overlapLeft === points.at(-1)!.x &&
+      to.y === points.at(-1)!.y
+    if (!sourceContact && !targetContact) return true
   }
   return false
 }
 
+function labelIntersectsBounds(label: FlowchartEdgeLabelLayout | undefined, bounds: FlowchartNodeBounds): boolean {
+  if (!label) return false
+  return (
+    label.point.x <= bounds.left + bounds.width - 1 &&
+    label.point.x + label.width - 1 >= bounds.left &&
+    label.point.y <= bounds.top + bounds.height - 1 &&
+    label.point.y + label.height - 1 >= bounds.top
+  )
+}
+
+function labelIntersectsSubgraphFrame(
+  label: FlowchartEdgeLabelLayout | undefined,
+  bounds: FlowchartSubgraphBounds,
+): boolean {
+  if (!label) return false
+  const labelRight = label.point.x + label.width - 1
+  const labelBottom = label.point.y + label.height - 1
+  const right = bounds.left + bounds.width - 1
+  const bottom = bounds.top + bounds.height - 1
+  return (
+    (label.point.x <= right &&
+      labelRight >= bounds.left &&
+      ((label.point.y <= bounds.top && labelBottom >= bounds.top) ||
+        (label.point.y <= bottom && labelBottom >= bottom))) ||
+    (label.point.y <= bottom &&
+      labelBottom >= bounds.top &&
+      ((label.point.x <= bounds.left && labelRight >= bounds.left) || (label.point.x <= right && labelRight >= right)))
+  )
+}
+
+function routeLength(route: FlowchartEdgeRoute): number {
+  let length = 0
+  for (let index = 1; index < route.points.length; index++) {
+    const from = route.points[index - 1]!
+    const to = route.points[index]!
+    length += Math.abs(to.x - from.x) + Math.abs(to.y - from.y)
+  }
+  return length
+}
+
+function labelIntersectsLabels(
+  label: FlowchartEdgeLabelLayout | undefined,
+  otherLabels: readonly FlowchartEdgeLabelLayout[],
+): boolean {
+  if (!label) return false
+  return otherLabels.some((otherLabel) => {
+    return label.lines.some((line, lineIndex) => {
+      const textLeft = label.point.x + 1
+      const textRight = label.point.x + diagramTextWidth(line) - 2
+      const y = label.point.y + lineIndex
+      return otherLabel.lines.some((otherLine, otherLineIndex) => {
+        const otherLeft = otherLabel.point.x
+        const otherRight = otherLeft + diagramTextWidth(otherLine) - 1
+        return y === otherLabel.point.y + otherLineIndex && textLeft <= otherRight && textRight >= otherLeft
+      })
+    })
+  })
+}
+
+function labelIntersectsLaterRoutePaths(
+  label: FlowchartEdgeLabelLayout | undefined,
+  laterRoutes: readonly FlowchartEdgeRoute[],
+): boolean {
+  if (!label) return false
+  return label.lines.some((line, lineIndex) => {
+    const width = diagramTextWidth(line) - 2
+    if (width <= 0) return false
+    return laterRoutes.some((other) =>
+      pathIntersectsBounds(other.points, {
+        left: label.point.x + 1,
+        top: label.point.y + lineIndex,
+        width,
+        height: 1,
+      }),
+    )
+  })
+}
+
 function avoidNodeObstacles(
   route: FlowchartEdgeRoute,
+  routes: readonly FlowchartEdgeRoute[],
   bounds: Map<string, FlowchartNodeBounds>,
-  direction: FlowchartDirection,
+  subgraphBounds: ReadonlyMap<string, FlowchartSubgraphBounds> | undefined,
+  routeIndex: number,
 ): FlowchartEdgeRoute {
-  const obstacle = [...bounds.values()].some(
-    (bound) => bound.id !== route.edge.from && bound.id !== route.edge.to && pathIntersectsBounds(route.points, bound),
+  const allNodeBounds = [...bounds.values()]
+  const allSubgraphBounds = [...(subgraphBounds?.values() ?? [])]
+  const laterRoutes = routes.slice(routeIndex + 1)
+  const laterLabels = laterRoutes.flatMap((laterRoute) =>
+    laterRoute.edge.label
+      ? [flowchartEdgeLabelLayout(laterRoute.points, laterRoute.edge.label, diagramTextWidth, laterRoute.labelAxis)]
+      : [],
   )
-  if (!obstacle) return route
+  const intersectsObstacle = (candidate: FlowchartEdgeRoute): boolean => {
+    const label = candidate.edge.label
+      ? flowchartEdgeLabelLayout(candidate.points, candidate.edge.label, diagramTextWidth, candidate.labelAxis)
+      : undefined
+    return (
+      allNodeBounds.some((bound) => {
+        const isSource = bound.id === route.edge.from
+        const isTarget = bound.id === route.edge.to
+        const allowedContact = isSource && isTarget ? "both" : isSource ? "source" : isTarget ? "target" : undefined
+        return pathIntersectsBounds(candidate.points, bound, allowedContact)
+      }) ||
+      allNodeBounds.some((bound) => labelIntersectsBounds(label, bound)) ||
+      allSubgraphBounds.some((bound) => labelIntersectsSubgraphFrame(label, bound)) ||
+      (subgraphBounds !== undefined &&
+        (labelIntersectsLabels(label, laterLabels) || labelIntersectsLaterRoutePaths(label, laterRoutes)))
+    )
+  }
+  if (!intersectsObstacle(route)) return route
 
   const from = bounds.get(route.edge.from)
   const to = bounds.get(route.edge.to)
   if (!from || !to) return route
-  if (isVerticalDirection(direction)) {
-    const start = boundsSidePoint(from, "right")
-    const end = boundsSidePoint(to, "right")
-    const busX = Math.max(...[...bounds.values()].map((bound) => bound.left + bound.width - 1)) + BUS_CLEARANCE
-    return { edge: route.edge, points: pathViaLane(start, lane("x", busX), end) }
+  const routingBounds = [...allNodeBounds, ...allSubgraphBounds]
+  const rightBusX = Math.max(...routingBounds.map((bound) => bound.left + bound.width - 1)) + BUS_CLEARANCE
+  const leftBusX = Math.min(...routingBounds.map((bound) => bound.left)) - BUS_CLEARANCE
+  const topBusY = Math.min(...routingBounds.map((bound) => bound.top)) - BUS_CLEARANCE
+  const bottomBusY = Math.max(...routingBounds.map((bound) => bound.top + bound.height - 1)) + BUS_CLEARANCE
+  const start = route.points[0]!
+  const end = route.points.at(-1)!
+  const targetSide = sideForOutsidePoint(to, end)
+  const approach = shiftPoint(
+    end,
+    targetSide === "left" ? "left" : targetSide === "right" ? "right" : targetSide === "top" ? "up" : "down",
+  )
+  const preservedTargetCandidates: FlowchartEdgeRoute[] = [
+    {
+      ...route,
+      labelAxis: route.labelAxis === undefined ? undefined : "y",
+      points: pathThrough([start, { x: leftBusX, y: start.y }, { x: leftBusX, y: approach.y }, approach, end]),
+    },
+    {
+      ...route,
+      labelAxis: route.labelAxis === undefined ? undefined : "y",
+      points: pathThrough([start, { x: rightBusX, y: start.y }, { x: rightBusX, y: approach.y }, approach, end]),
+    },
+    {
+      ...route,
+      labelAxis: route.labelAxis === undefined ? undefined : "x",
+      points: pathThrough([start, { x: start.x, y: topBusY }, { x: approach.x, y: topBusY }, approach, end]),
+    },
+    {
+      ...route,
+      labelAxis: route.labelAxis === undefined ? undefined : "x",
+      points: pathThrough([start, { x: start.x, y: bottomBusY }, { x: approach.x, y: bottomBusY }, approach, end]),
+    },
+  ]
+  const candidates: FlowchartEdgeRoute[] = [
+    {
+      ...route,
+      labelAxis: route.labelAxis === undefined ? undefined : "y",
+      points: pathViaLane(boundsSidePoint(from, "right"), lane("x", rightBusX), boundsSidePoint(to, "right")),
+    },
+    {
+      ...route,
+      labelAxis: route.labelAxis === undefined ? undefined : "y",
+      points: pathViaLane(boundsSidePoint(from, "left"), lane("x", leftBusX), boundsSidePoint(to, "left")),
+    },
+    {
+      ...route,
+      labelAxis: route.labelAxis === undefined ? undefined : "x",
+      points: pathViaLane(boundsSidePoint(from, "top"), lane("y", topBusY), boundsSidePoint(to, "top")),
+    },
+    {
+      ...route,
+      labelAxis: route.labelAxis === undefined ? undefined : "x",
+      points: pathViaLane(boundsSidePoint(from, "bottom"), lane("y", bottomBusY), boundsSidePoint(to, "bottom")),
+    },
+  ]
+  const shortestValid = (candidateRoutes: FlowchartEdgeRoute[]): FlowchartEdgeRoute | undefined =>
+    candidateRoutes
+      .filter((candidate) => !intersectsObstacle(candidate))
+      .sort((left, right) => routeLength(left) - routeLength(right))[0]
+  if (subgraphBounds) {
+    return shortestValid(preservedTargetCandidates) ?? shortestValid(candidates) ?? route
   }
-
-  const start = boundsSidePoint(from, "top")
-  const end = boundsSidePoint(to, "top")
-  const busY = Math.min(...[...bounds.values()].map((bound) => bound.top)) - BUS_CLEARANCE
-  return { edge: route.edge, points: pathViaLane(start, lane("y", busY), end) }
+  return (
+    candidates.find((candidate) => !intersectsObstacle(candidate)) ?? shortestValid(preservedTargetCandidates) ?? route
+  )
 }
 
 export function routeFlowchartEdges(
@@ -671,7 +865,10 @@ export function routeFlowchartEdges(
     if (!from || !to) continue
     routes.push({ edge, points: edgePath(from, to, directionForEdge(edge), leftBoundary) })
   }
-  return routes.map((route) => avoidNodeObstacles(route, bounds, directionForEdge(route.edge)))
+  for (let index = routes.length - 1; index >= 0; index--) {
+    routes[index] = avoidNodeObstacles(routes[index]!, routes, bounds, subgraphBounds, index)
+  }
+  return routes
 }
 
 function sideForOutsidePoint(bounds: FlowchartNodeBounds, sourcePoint: FlowchartPoint): DiagramSide {

packages/merman/src/flowchart/types.ts

@@ -1,4 +1,4 @@
-import type { DiagramBounds, DiagramDirection, DiagramPoint } from "../core/geometry.js"
+import type { DiagramAxis, DiagramBounds, DiagramDirection, DiagramPoint } from "../core/geometry.js"
 
 export type FlowchartDirection = "TB" | "TD" | "BT" | "LR" | "RL"
 export type FlowchartNodeShape = "box" | "rounded" | "database" | "decision" | "subroutine"
@@ -16,6 +16,7 @@ export interface FlowchartEdge {
   label: string
   style?: FlowchartEdgeStyle
   arrowhead?: false
+  sourceArrowhead?: true
   orderOnly?: boolean
 }
 
@@ -55,6 +56,7 @@ export type FlowchartPoint = DiagramPoint
 export interface FlowchartEdgeRoute {
   edge: FlowchartEdge
   points: FlowchartPoint[]
+  labelAxis?: DiagramAxis
 }
 
 export type FlowchartEdgeDirection = DiagramDirection

packages/merman/src/sequence/diagram.test.ts

@@ -1,4 +1,5 @@
 import { describe, expect, test } from "bun:test"
+import { diagramTextWidth } from "../core/text.js"
 import { expectDiagram } from "../test/diagram.js"
 import { renderSequenceDiagram } from "./diagram.js"
 import { drawSequenceDiagramGrid } from "./drawing.js"
@@ -28,6 +29,18 @@ sequenceDiagram
     ])
   })
 
+  test("decodes HTML entities in participant, message, and note labels", () => {
+    const diagram = parseMermaidSequenceDiagram(`sequenceDiagram
+  participant A as Worker & signer
+  participant B
+  A->>B: ack <3s
+  Note over A,B: result ≥ 1`)
+
+    expect(diagram.participants[0]?.label).toBe("Worker & signer")
+    expect(diagram.messages[0]?.label).toBe("ack <3s")
+    expect(diagram.steps.find((step) => step.type === "note")?.note.label).toBe("result ≥ 1")
+  })
+
   test("renders a terminal sequence diagram", () => {
     const output = renderSequenceDiagram(`
 sequenceDiagram
@@ -42,11 +55,11 @@ sequenceDiagram
       │ Browser │       │ Server │
       ╰────┬────╯       ╰────┬───╯
            │                 │
-           │ GET /           │
-           ├─────────────────▶
+           │     GET /       │
+           ├─────────────────►
            │                 │
-           │ 401 WWW-Auth    │
-           ◀─────────────────┤
+           │  401 WWW-Auth   │
+           ◄─────────────────┤
            │                 │
     `)
   })
@@ -70,15 +83,15 @@ sequenceDiagram
     expectDiagram(output).toEqualDiagram(`
       leaf tool                       LocationMutation             FileMutation
           │                                   │                          │
-          ├─ resolve(path) ───────────────────▶                          │
+          ├───────── resolve(path) ───────────►                          │
           │                                   │                          │
-          ◀─ Plan(target, authority anchor) ──┤                          │
+          ◄─ Plan(target, authority anchor) ──┤                          │
           │                                   │                          │
-          ├─ commit(plan) ───────────────────────────────────────────────▶
+          ├─────────────────────── commit(plan) ─────────────────────────►
           │                                   │                          │
-          │                                   ◀─ revalidate(plan) ───────┤
+          │                                   ◄─── revalidate(plan) ─────┤
           │                                   │                          │
-          │                                   ├─ same target or reject ──▶
+          │                                   ├─ same target or reject ──►
           │                                   │                          │
     `)
   })
@@ -109,7 +122,7 @@ sequenceDiagram
     const lines = output.split("\n")
 
     expect(lines.findIndex((line) => line.includes("deliberately"))).toBeLessThan(
-      lines.findIndex((line) => line.includes("▶")),
+      lines.findIndex((line) => line.includes("►")),
     )
   })
 
@@ -245,18 +258,29 @@ sequenceDiagram
     ])
   })
 
-  test("parses activation syntax without rendering activation bars", () => {
+  test("renders activation syntax as visible intervals", () => {
     const output = renderSequenceDiagram(`
 sequenceDiagram
   Browser->>+Server: request
   Server-->>-Browser: response
 `)
 
-    expect(output).not.toContain("┃")
+    expect(output).toContain("┃")
     expect(output).toContain("request")
     expect(output).toContain("response")
   })
 
+  test("renders br-delimited participant aliases on separate lines", () => {
+    const output = renderSequenceDiagram(`sequenceDiagram
+  participant A as First line<br/>Second line
+  participant B as Normal
+  A->>B: hello`)
+
+    expect(output).not.toContain("<br")
+    expect(output).toContain("│ First line  │")
+    expect(output).toContain("│ Second line │")
+  })
+
   test("parses Mermaid arrow head variants", () => {
     const diagram = parseMermaidSequenceDiagram(`
 sequenceDiagram
@@ -294,22 +318,22 @@ sequenceDiagram
       │ A │              │ B │
       ╰─┬─╯              ╰─┬─╯
         │                  │
-        │ open solid       │
+        │   open solid     │
         ├─────────────────>│
         │                  │
-        │ open dashed      │
+        │   open dashed    │
         │<─────────────────┤
         │                  │
-        │ failed solid     │
+        │  failed solid    │
         ├─────────────────✕│
         │                  │
-        │ failed dashed    │
+        │  failed dashed   │
         │✕─────────────────┤
         │                  │
-        │ async solid      │
+        │   async solid    │
         ├─────────────────)│
         │                  │
-        │ async dashed     │
+        │  async dashed    │
         │(─────────────────┤
         │                  │"
     `)
@@ -533,7 +557,7 @@ sequenceDiagram
     const fragmentMessageRow = fragment.split("\n").find((line) => line.includes("this non adjacent message"))!
     expect(groupMessageRow.trimEnd().endsWith("│")).toBe(true)
     expect(fragmentMessageRow).toContain("this non adjacent message is deliberately much wider than the frame")
-    expect(fragmentMessageRow.match(/│/g)?.length).toBe(3)
+    expect(fragmentMessageRow.match(/│/g)?.length).toBe(2)
   })
 
   test("keeps long notes inside groups and nested fragment frames intact", () => {
@@ -581,6 +605,42 @@ sequenceDiagram
     expect(externalHeaderLeft).toBeGreaterThan(groupBorderRight)
   })
 
+  test("keeps adjacent wide participant group frames separate", () => {
+    const output = renderSequenceDiagram(
+      `sequenceDiagram
+  box First very wide group heading
+    participant A
+  end
+  box Second very wide group heading
+    participant B
+  end
+  A->>B: hi`,
+      { compact: true },
+    )
+    const topRow = output.split("\n")[0]!
+
+    expect(topRow).toContain("First very wide group heading")
+    expect(topRow).toContain("Second very wide group heading")
+    expect(topRow.indexOf("╮")).toBeLessThan(topRow.lastIndexOf("╭"))
+  })
+
+  test("renders many adjacent wide participant groups without excessive canvas growth", () => {
+    const groupCount = 16
+    const output = renderSequenceDiagram(
+      `sequenceDiagram
+${Array.from(
+  { length: groupCount },
+  (_, index) => `  box Group ${index} has a deliberately wide heading
+    participant P${index}
+  end`,
+).join("\n")}
+  P0->>P15: hi`,
+      { compact: true },
+    )
+
+    expect(Math.max(...output.split("\n").map(diagramTextWidth))).toBeLessThan(groupCount * 60)
+  })
+
   test("renders full-height participant group boxes", () => {
     const output = renderSequenceDiagram(`
 sequenceDiagram
@@ -600,11 +660,11 @@ sequenceDiagram
       │ Browser │        │ │ API │          │ Cache │          │ DB │ │
       ╰────┬────╯        │ ╰──┬──╯          ╰───┬───╯          ╰──┬─╯ │
            │             │    │                 │                 │   │
-           │ GET /users/42    │                 │                 │   │
-           ├──────────────────▶                 │                 │   │
+           │  GET /users/42   │                 │                 │   │
+           ├──────────────────►                 │                 │   │
            │             │    │                 │                 │   │
-           │             │    │ get user:42     │                 │   │
-           │             │    ├─────────────────▶                 │   │
+           │             │    │  get user:42    │                 │   │
+           │             │    ├─────────────────►                 │   │
            │             │    │                 │                 │   │
                          ╰────────────────────────────────────────────╯"
     `)
@@ -619,12 +679,25 @@ sequenceDiagram
   end
   Browser->>API: GET /users/42
 `)
-    const arrowLine = output.split("\n").find((line) => line.includes("▶"))!
+    const arrowLine = output.split("\n").find((line) => line.includes("►"))!
 
-    expect(arrowLine).toContain("───────────────▶")
+    expect(arrowLine).toContain("───────────────►")
     expect(arrowLine).not.toContain("┼")
   })
 
+  test("keeps filled arrowheads to one terminal column", () => {
+    const output = renderSequenceDiagram(`sequenceDiagram
+  box Backend
+    participant A
+    participant B
+    A->>B: request
+  end`)
+    const lines = output.split("\n")
+    const frameWidth = diagramTextWidth(lines.at(-1)!)
+
+    expect(Math.max(...lines.map(diagramTextWidth))).toBe(frameWidth)
+  })
+
   test("renders self messages as loopback arrows", () => {
     const output = renderSequenceDiagram(`
 sequenceDiagram
@@ -639,12 +712,12 @@ sequenceDiagram
            │
            ├────────────────────╮
            │ Check Permissions  │
-           ◀────────────────────╯
+           ◄────────────────────╯
            │"
     `)
   })
 
-  test("places two spacer rows above note badges and one below", () => {
+  test("frames notes in their reserved rows", () => {
     const output = renderSequenceDiagram(`
 sequenceDiagram
   Browser->>Server: one
@@ -656,9 +729,11 @@ sequenceDiagram
     const nextMessageRow = lines.findIndex((line) => line.includes("two"))
 
     expect(noteRow).toBeGreaterThan(0)
-    expect(lines[noteRow - 1]?.trim()).toBe("│                 │")
-    expect(lines[noteRow - 2]?.trim()).toBe("│                 │")
-    expect(lines[noteRow + 1]?.trim()).toBe("│                 │")
+    expect(lines[noteRow - 1]).toContain("╭")
+    expect(lines[noteRow - 1]).toContain("╮")
+    expect(lines[noteRow]).toContain("│ phase │")
+    expect(lines[noteRow + 1]).toContain("╰")
+    expect(lines[noteRow + 1]).toContain("╯")
     expect(nextMessageRow).toBe(noteRow + 2)
   })
 

packages/merman/src/sequence/drawing.ts

@@ -1,5 +1,6 @@
 import { BorderChars, type BorderStyle } from "@opentui/core"
 import { DiagramCanvas } from "../core/canvas.js"
+import { diagramTextWidth } from "../core/text.js"
 import { DEFAULT_FRAGMENT_BORDER_STYLE } from "./options.js"
 import {
   createSequencePlacementPlan,
@@ -19,6 +20,10 @@ import type {
 
 const SEQUENCE_BORDER = BorderChars.rounded
 
+function centeredStart(center: number, text: string): number {
+  return center - Math.floor(diagramTextWidth(text) / 2)
+}
+
 function arrowHeadChar(head: SequenceArrowHead | undefined, direction: 1 | -1): string {
   switch (head) {
     case "open":
@@ -28,7 +33,7 @@ function arrowHeadChar(head: SequenceArrowHead | undefined, direction: 1 | -1):
     case "async":
       return direction === 1 ? ")" : "("
     default:
-      return direction === 1 ? "▶" : "◀"
+      return direction === 1 ? "►" : "◄"
   }
 }
 
@@ -185,6 +190,32 @@ function renderSelfMessage(
   setCell(grid, rightX, bottomRow, SEQUENCE_BORDER.bottomRight, style)
 }
 
+function renderNote(grid: SequenceGrid, placement: Extract<SequenceStepPlacement, { type: "note" }>): void {
+  const width = Math.max(...placement.textLines.map(diagramTextWidth))
+  const left = placement.textX
+  const right = left + width - 1
+  const top = placement.textY - 1
+  const bottom = placement.textY + placement.textLines.length
+
+  for (let x = left + 1; x < right; x++) {
+    setCell(grid, x, top, SEQUENCE_BORDER.horizontal, "note")
+    setCell(grid, x, bottom, SEQUENCE_BORDER.horizontal, "note")
+  }
+  for (let y = top + 1; y < bottom; y++) {
+    setCell(grid, left, y, SEQUENCE_BORDER.vertical, "note")
+    setCell(grid, right, y, SEQUENCE_BORDER.vertical, "note")
+  }
+  setCell(grid, left, top, SEQUENCE_BORDER.topLeft, "note")
+  setCell(grid, right, top, SEQUENCE_BORDER.topRight, "note")
+  setCell(grid, left, bottom, SEQUENCE_BORDER.bottomLeft, "note")
+  setCell(grid, right, bottom, SEQUENCE_BORDER.bottomRight, "note")
+  placement.textLines.forEach((line, index) => setText(grid, left, placement.textY + index, line, "noteBadge"))
+  for (let y = placement.textY; y < bottom; y++) {
+    setCell(grid, left, y, SEQUENCE_BORDER.vertical, "note")
+    setCell(grid, right, y, SEQUENCE_BORDER.vertical, "note")
+  }
+}
+
 export function drawSequenceDiagramGrid(
   diagram: SequenceDiagram,
   options: SequenceDiagramRenderOptions = {},
@@ -197,11 +228,13 @@ export function drawSequenceDiagramGrid(
   if (plan.groups.length > 0) renderParticipantGroups(grid, plan.groups, plan.height - 1)
 
   for (const placement of plan.participants) {
-    const { participant, centerX: center, headerLeftX, headerRightX, labelX } = placement
+    const { centerX: center, headerLeftX, headerRightX, labelLines } = placement
     const { participantHeaderTopY, participantHeaderY, participantRuleY, lifelineStartY, lifelineEndY } = plan.rows
 
     if (options.compact) {
-      setText(grid, labelX, participantHeaderY, participant.label, "participant")
+      labelLines.forEach((line, index) =>
+        setText(grid, centeredStart(center, line), participantHeaderY + index, line, "participant"),
+      )
     } else {
       for (let x = headerLeftX; x <= headerRightX; x++) {
         setCell(grid, x, participantHeaderTopY, SEQUENCE_BORDER.horizontal, "participant")
@@ -210,11 +243,15 @@ export function drawSequenceDiagramGrid(
 
       setCell(grid, headerLeftX, participantHeaderTopY, SEQUENCE_BORDER.topLeft, "participant")
       setCell(grid, headerRightX, participantHeaderTopY, SEQUENCE_BORDER.topRight, "participant")
-      setCell(grid, headerLeftX, participantHeaderY, SEQUENCE_BORDER.vertical, "participant")
-      setCell(grid, headerRightX, participantHeaderY, SEQUENCE_BORDER.vertical, "participant")
+      for (let y = participantHeaderY; y < participantRuleY; y++) {
+        setCell(grid, headerLeftX, y, SEQUENCE_BORDER.vertical, "participant")
+        setCell(grid, headerRightX, y, SEQUENCE_BORDER.vertical, "participant")
+      }
       setCell(grid, headerLeftX, participantRuleY, SEQUENCE_BORDER.bottomLeft, "participant")
       setCell(grid, headerRightX, participantRuleY, SEQUENCE_BORDER.bottomRight, "participant")
-      setText(grid, labelX, participantHeaderY, participant.label, "participant")
+      labelLines.forEach((line, index) =>
+        setText(grid, centeredStart(center, line), participantHeaderY + index, line, "participant"),
+      )
       setCell(grid, center, participantRuleY, SEQUENCE_BORDER.topT, "participant")
     }
 
@@ -227,9 +264,7 @@ export function drawSequenceDiagramGrid(
 
   for (const placement of plan.steps) {
     if (placement.type === "note") {
-      for (let lineIndex = 0; lineIndex < placement.textLines.length; lineIndex++) {
-        setText(grid, placement.textX, placement.textY + lineIndex, placement.textLines[lineIndex]!, "noteBadge")
-      }
+      renderNote(grid, placement)
       continue
     }
 
@@ -270,5 +305,13 @@ export function drawSequenceDiagramGrid(
     if (placement.inlineLabel) setText(grid, placement.labelX, placement.labelY, placement.inlineLabel, messageStyle)
   }
 
+  for (const activation of plan.activations) {
+    for (let y = activation.startY; y <= activation.endY; y++) {
+      if (grid.getCell(activation.centerX, y)?.char === SEQUENCE_BORDER.vertical) {
+        setCell(grid, activation.centerX, y, "┃", "lifeline")
+      }
+    }
+  }
+
   return grid
 }

packages/merman/src/sequence/parser.ts

@@ -22,6 +22,7 @@ const ALT_RE = /^alt\s+(.+)$/i
 const ELSE_RE = /^else(?:\s+(.+))?$/i
 const LOOP_RE = /^loop\s+(.+)$/i
 const AUTONUMBER_RE = /^autonumber(?:\s+(\d+)(?:\s+(\d+))?)?$/i
+const UNSUPPORTED_BIDIRECTIONAL_MESSAGE_RE = /<<-{1,2}>>/
 const CSS_COLOR_NAMES = new Set([
   "black",
   "white",
@@ -132,6 +133,9 @@ export function parseMermaidSequenceDiagram(content: string): SequenceDiagram {
   for (const source of meaningfulNumberedMermaidLines(content)) {
     const line = source.text
     if (line.toLowerCase() === "sequencediagram") continue
+    if (UNSUPPORTED_BIDIRECTIONAL_MESSAGE_RE.test(line)) {
+      throw new MermaidSyntaxError("sequence", source.lineNumber, line)
+    }
 
     const autonumberMatch = line.match(AUTONUMBER_RE)
     if (autonumberMatch) {

packages/merman/src/sequence/placement.test.ts

@@ -90,6 +90,25 @@ describe("createSequencePlacementPlan", () => {
     expect(external.headerLeftX).toBeGreaterThan(group.rightX)
   })
 
+  test("keeps many adjacent wide groups at a linear width", () => {
+    const groupCount = 16
+    const source = `sequenceDiagram
+${Array.from(
+  { length: groupCount },
+  (_, index) => `  box Group ${index} has a deliberately wide heading
+    participant P${index}
+  end`,
+).join("\n")}
+  P0->>P15: hi`
+    const plan = createSequencePlacementPlan(parseMermaidSequenceDiagram(source), { compact: true })
+
+    expect(plan.groups).toHaveLength(groupCount)
+    for (let index = 1; index < plan.groups.length; index++) {
+      expect(plan.groups[index]!.leftX).toBeGreaterThan(plan.groups[index - 1]!.rightX)
+    }
+    expect(plan.width).toBeLessThan(groupCount * 60)
+  })
+
   test("expands group and fragment frames around contained long content", () => {
     const groupPlan = createSequencePlacementPlan(
       parseMermaidSequenceDiagram(`sequenceDiagram
@@ -169,4 +188,34 @@ describe("createSequencePlacementPlan", () => {
 
     expect(starts[0]!.bounds.rightX).toBeGreaterThan(starts[1]!.bounds.rightX)
   })
+
+  test("aligns explicit and shorthand activation intervals to message events", () => {
+    const shorthand = createSequencePlacementPlan(
+      parseMermaidSequenceDiagram(`sequenceDiagram
+  A->>+B: request
+  B-->>-A: response`),
+    )
+    const explicit = createSequencePlacementPlan(
+      parseMermaidSequenceDiagram(`sequenceDiagram
+  A->>B: request
+  activate B
+  B-->>A: response
+  deactivate B`),
+    )
+
+    expect(explicit.activations).toEqual(shorthand.activations)
+  })
+
+  test("centers message label blocks over their arrow span", () => {
+    const plan = createSequencePlacementPlan(
+      parseMermaidSequenceDiagram(`sequenceDiagram
+  participant A
+  participant B
+  A->>B: short<br/>a much longer line`),
+    )
+    const message = plan.steps.find((step) => step.type === "message")!
+    const labelWidth = Math.max(...message.labelLines.map(diagramTextWidth))
+
+    expect(message.labelX * 2 + labelWidth).toBe(message.leftX + message.rightX)
+  })
 })

packages/merman/src/sequence/placement.ts

@@ -11,7 +11,7 @@ import type {
   SequenceStep,
 } from "./types.js"
 
-const NOTE_HORIZONTAL_PADDING = 1
+const NOTE_HORIZONTAL_PADDING = 2
 const GROUP_HORIZONTAL_PADDING = 2
 const FRAGMENT_HORIZONTAL_OVERHANG = 3
 
@@ -25,7 +25,7 @@ export interface SequenceParticipantPlacement {
   centerX: number
   headerLeftX: number
   headerRightX: number
-  labelX: number
+  labelLines: string[]
 }
 
 export interface SequenceGroupPlacement {
@@ -41,6 +41,14 @@ export interface SequenceWallPlacement {
   endY: number
 }
 
+export interface SequenceActivationPlacement {
+  participant: string
+  centerX: number
+  startY: number
+  endY: number
+  depth: number
+}
+
 export type SequenceStepPlacement =
   | { type: "note"; note: SequenceNote; textLines: string[]; textX: number; textY: number }
   | {
@@ -88,6 +96,7 @@ export interface SequencePlacementPlan {
   }
   participants: SequenceParticipantPlacement[]
   groups: SequenceGroupPlacement[]
+  activations: SequenceActivationPlacement[]
   steps: SequenceStepPlacement[]
 }
 
@@ -132,7 +141,8 @@ function messageLabelText(message: SequenceMessage): string {
 }
 
 function participantHeaderWidth(label: string, compact: boolean): number {
-  return compact ? visualLength(label) : Math.max(5, visualLength(label) + 4)
+  const width = labelLinesWidth(mermaidLabelLines(label))
+  return compact ? width : Math.max(5, width + 4)
 }
 
 function fragmentLabelText(fragment: SequenceFragment): string {
@@ -236,7 +246,9 @@ function getStepContentBounds(
     if (fromIndex === toIndex) return { leftX: fromX, rightX: fromX + selfMessageLoopWidth(step.message) }
     const leftX = Math.min(fromX, toX)
     const rightX = Math.max(fromX, toX)
-    return { leftX, rightX: Math.max(rightX, leftX + 2 + messageWidth(step.message) - 1) }
+    const labelWidth = messageWidth(step.message)
+    const labelLeftX = Math.floor((leftX + rightX - labelWidth) / 2)
+    return { leftX: Math.min(leftX, labelLeftX), rightX: Math.max(rightX, labelLeftX + labelWidth - 1) }
   }
   if (step.type !== "note") return undefined
   const indexes = getParticipantIndexes(participantIndexes, step.note.over)
@@ -387,7 +399,9 @@ function resolveParticipantCenters(
     if (fromIndex === toIndex && fromIndex >= 0 && fromIndex < diagram.participants.length - 1) {
       gaps[fromIndex] = Math.max(
         gaps[fromIndex]!,
-        selfMessageLoopWidth(message) + Math.ceil(visualLength(diagram.participants[fromIndex + 1]!.label) / 2) + 2,
+        selfMessageLoopWidth(message) +
+          Math.ceil(labelLinesWidth(mermaidLabelLines(diagram.participants[fromIndex + 1]!.label)) / 2) +
+          2,
       )
       continue
     }
@@ -423,37 +437,31 @@ function separateExpandedGroupsFromExternalParticipants(
   compact: boolean,
 ): number[] {
   const adjusted = [...centers]
-  for (let pass = 0; pass < Math.max(1, ranges.length * 2); pass++) {
-    let changed = false
+  for (let boundary = 0; boundary < adjusted.length - 1; boundary++) {
     const groups = resolveGroupBounds(diagram, adjusted, participantIndexes, ranges, compact)
+    const leftWidth = participantHeaderWidth(diagram.participants[boundary]!.label, compact)
+    const rightWidth = participantHeaderWidth(diagram.participants[boundary + 1]!.label, compact)
+    let leftRight = adjusted[boundary]! - Math.floor(leftWidth / 2) + leftWidth - 1
+    let rightLeft = adjusted[boundary + 1]! - Math.floor(rightWidth / 2)
+    let bordersGroup = false
+
     for (const [index, range] of ranges.entries()) {
-      const group = groups[index]!
-      if (range.startIndex > 0) {
-        const previousIndex = range.startIndex - 1
-        const previousWidth = participantHeaderWidth(diagram.participants[previousIndex]!.label, compact)
-        const previousRight = adjusted[previousIndex]! - Math.floor(previousWidth / 2) + previousWidth - 1
-        const shift = previousRight + GROUP_HORIZONTAL_PADDING + 1 - group.leftX
-        if (shift > 0) {
-          for (let participantIndex = range.startIndex; participantIndex < adjusted.length; participantIndex++) {
-            adjusted[participantIndex]! += shift
-          }
-          changed = true
-        }
+      if (range.endIndex === boundary) {
+        leftRight = Math.max(leftRight, groups[index]!.rightX)
+        bordersGroup = true
       }
-      if (range.endIndex < diagram.participants.length - 1) {
-        const nextIndex = range.endIndex + 1
-        const nextWidth = participantHeaderWidth(diagram.participants[nextIndex]!.label, compact)
-        const nextLeft = adjusted[nextIndex]! - Math.floor(nextWidth / 2)
-        const shift = group.rightX + GROUP_HORIZONTAL_PADDING + 1 - nextLeft
-        if (shift > 0) {
-          for (let participantIndex = nextIndex; participantIndex < adjusted.length; participantIndex++) {
-            adjusted[participantIndex]! += shift
-          }
-          changed = true
-        }
+      if (range.startIndex === boundary + 1) {
+        rightLeft = Math.min(rightLeft, groups[index]!.leftX)
+        bordersGroup = true
       }
     }
-    if (!changed) return adjusted
+
+    if (!bordersGroup) continue
+    const shift = leftRight + GROUP_HORIZONTAL_PADDING + 1 - rightLeft
+    if (shift <= 0) continue
+    for (let participantIndex = boundary + 1; participantIndex < adjusted.length; participantIndex++) {
+      adjusted[participantIndex]! += shift
+    }
   }
   return adjusted
 }
@@ -475,6 +483,7 @@ export function createSequencePlacementPlan(
       },
       participants: [],
       groups: [],
+      activations: [],
       steps: [],
     }
   }
@@ -511,9 +520,13 @@ export function createSequencePlacementPlan(
     fragments = fragmentBounds()
   }
   const hasGroups = groups.length > 0
+  const participantLabelHeight = Math.max(
+    1,
+    ...diagram.participants.map((participant) => mermaidLabelLines(participant.label).length),
+  )
   const participantHeaderTopY = hasGroups ? 1 : 0
   const participantHeaderY = participantHeaderTopY + (compact ? 0 : 1)
-  const participantRuleY = participantHeaderTopY + (compact ? 0 : 2)
+  const participantRuleY = participantHeaderTopY + (compact ? participantLabelHeight - 1 : participantLabelHeight + 1)
   const lifelineStartY = participantRuleY + 1
   const stepStartY = lifelineStartY + 1
   const width = Math.max(contentBounds.rightX + 1, ...groups.map((group) => group.rightX + 1), fragments.rightX + 1)
@@ -525,19 +538,46 @@ export function createSequencePlacementPlan(
     const centerX = centers[index]!
     const width = participantHeaderWidth(participant.label, compact)
     const headerLeftX = centerX - Math.floor(width / 2)
+    const labelLines = mermaidLabelLines(participant.label)
     return {
       participant,
       centerX,
       headerLeftX,
       headerRightX: headerLeftX + width - 1,
-      labelX: centeredStart(centerX, participant.label),
+      labelLines,
     }
   })
   const steps: SequenceStepPlacement[] = []
+  const activations: SequenceActivationPlacement[] = []
+  const activeByParticipant = new Map<string, Array<{ startY: number; depth: number }>>()
+  const lastEventYByParticipant = new Map<string, number>()
+  const openActivation = (participant: string, y: number): void => {
+    const active = activeByParticipant.get(participant) ?? []
+    active.push({ startY: y, depth: active.length })
+    activeByParticipant.set(participant, active)
+  }
+  const closeActivation = (participant: string, y: number): void => {
+    const active = activeByParticipant.get(participant)
+    const opened = active?.pop()
+    const participantIndex = indexes.get(participant)
+    if (!opened || participantIndex === undefined) return
+    activations.push({
+      participant,
+      centerX: centers[participantIndex]!,
+      startY: opened.startY,
+      endY: y,
+      depth: opened.depth,
+    })
+  }
   let stepY = stepStartY
   const activeFrames: ActiveFragmentFrame[] = []
   for (const [stepIndex, step] of diagram.steps.entries()) {
-    if (step.type === "activation") continue
+    if (step.type === "activation") {
+      const eventY = Math.min(lastEventYByParticipant.get(step.activation.participant) ?? stepY, lifelineEndY)
+      if (step.activation.active) openActivation(step.activation.participant, eventY)
+      else closeActivation(step.activation.participant, eventY)
+      continue
+    }
     const stepHeight = getStepHeight(step, centers, indexes, compact)
     if (step.type === "note") {
       const noteIndexes = getParticipantIndexes(indexes, step.note.over)
@@ -588,6 +628,7 @@ export function createSequencePlacementPlan(
     const labelLines = messageLabelLines(messageLabelText(step.message))
     if (fromIndex === toIndex) {
       const centerX = centers[fromIndex]!
+      const bottomY = stepY + labelLines.length + 1
       steps.push({
         type: "selfMessage",
         message: step.message,
@@ -595,8 +636,11 @@ export function createSequencePlacementPlan(
         centerX,
         rightX: centerX + selfMessageLoopWidthForLines(labelLines),
         topY: stepY,
-        bottomY: stepY + labelLines.length + 1,
+        bottomY,
       })
+      if (step.message.activate) openActivation(step.message.activate, bottomY)
+      if (step.message.deactivate) closeActivation(step.message.deactivate, bottomY)
+      lastEventYByParticipant.set(step.message.from, bottomY)
     } else {
       const fromX = centers[fromIndex]!
       const toX = centers[toIndex]!
@@ -604,13 +648,16 @@ export function createSequencePlacementPlan(
       const leftX = Math.min(fromX, toX)
       const rightX = Math.max(fromX, toX)
       const inlineLabel = inlineMessageLabel(step.message, labelLines, fromX, toX, compact)
+      const arrowY = inlineLabel ? stepY : stepY + labelLines.length
+      const renderedLabelWidth = inlineLabel ? visualLength(inlineLabel) : labelLinesWidth(labelLines)
+      const labelX = Math.floor((leftX + rightX - renderedLabelWidth) / 2)
       steps.push({
         type: "message",
         message: step.message,
         labelLines,
-        labelX: leftX + 2,
+        labelX,
         labelY: stepY,
-        arrowY: inlineLabel ? stepY : stepY + labelLines.length,
+        arrowY,
         fromX,
         toX,
         leftX,
@@ -619,15 +666,23 @@ export function createSequencePlacementPlan(
         headX: arrowHeadX(toX, direction, step.message.head),
         inlineLabel,
       })
+      if (step.message.activate) openActivation(step.message.activate, arrowY)
+      if (step.message.deactivate) closeActivation(step.message.deactivate, arrowY)
+      lastEventYByParticipant.set(step.message.from, arrowY)
+      lastEventYByParticipant.set(step.message.to, arrowY)
     }
     stepY += stepHeight
   }
+  for (const [participant, active] of activeByParticipant) {
+    while (active.length > 0) closeActivation(participant, lifelineEndY)
+  }
   return {
     width,
     height,
     rows: { participantHeaderTopY, participantHeaderY, participantRuleY, lifelineStartY, lifelineEndY },
     participants,
     groups,
+    activations,
     steps,
   }
 }

packages/merman/src/state/diagram.test.ts

@@ -47,6 +47,17 @@ stateDiagram-v2
     })
   })
 
+  test("decodes HTML entities in state, transition, and note labels", () => {
+    const diagram = parseMermaidStateDiagram(`stateDiagram-v2
+  state "Ready & waiting" as Ready
+  Ready --> Done: elapsed <3s
+  note right of Done: result ≥ 1`)
+
+    expect(diagram.states.find((state) => state.id === "Ready")?.label).toBe("Ready & waiting")
+    expect(diagram.transitions[0]?.label).toBe("elapsed <3s")
+    expect(diagram.notes[0]?.lines).toEqual(["result ≥ 1"])
+  })
+
   test("parses choice pseudo-states", () => {
     const diagram = parseMermaidStateDiagram(`
 stateDiagram-v2
@@ -55,7 +66,7 @@ stateDiagram-v2
   Decision --> Accepted: yes
 `)
 
-    expect(diagram.states).toContainEqual({ id: "Decision", label: "┼", kind: "choice" })
+    expect(diagram.states).toContainEqual({ id: "Decision", label: "", kind: "choice" })
   })
 
   test("parses composite states and notes", () => {
@@ -188,7 +199,7 @@ stateDiagram-v2
       ●───────────────────────▶│ Running │
                                ╰──┬──────╯ 💥 sandbox dies BEFORE hook fires
                                 ▲ │   ▲    (crash, our bug, race)
-                       ╭────────┼─╰───┼───────╮
+                       ╭────────┼─┴───┼───────╮
                        ▼   ╭────┼─────╯       ▼
                     ╭──────┴──╮ │           ╭──────╮
                     │ Dormant │ │           │ Lost │
@@ -384,7 +395,7 @@ stateDiagram-v2
 
     expect(output).toMatchInlineSnapshot(`
       "              ╭─────────╮   submit          ok      ╭───────╮
-      ●────────────▶│ Editing ├─────────────┬────────────▶│ Saved │
+      ●────────────▶│ Editing ├────────────▶◆────────────▶│ Saved │
                     ╰──┬──────╯             │             ╰───────╯
                      ▲ │    ▲ type          │ fail
                      │ ╰────╯               │
@@ -411,7 +422,7 @@ stateDiagram-v2
   Decision --> Done
   Done --> [*]`)
 
-    expect(output).toContain("Upper ├─────────────┬────────────▶│ Done")
+    expect(output).toContain("Upper ├────────────▶◆────────────▶│ Done")
   })
 
   test("renders self transitions as loops in vertical diagrams", () => {
@@ -444,6 +455,65 @@ stateDiagram-v2
     expect(vertical).toContain("second")
   })
 
+  test("separates labels on four parallel vertical transitions", () => {
+    const output = renderStateDiagram(`stateDiagram-v2
+  direction TB
+  A --> B: one
+  A --> B: two
+  A --> B: three
+  A --> B: four`)
+
+    expect(output).not.toContain("twothree")
+    for (const label of ["one", "two", "three", "four"]) {
+      expect(output.match(new RegExp(label, "g"))).toHaveLength(1)
+    }
+  })
+
+  test("keeps explicit choices visible in choice-only cycles", () => {
+    const output = renderStateDiagram(`stateDiagram-v2
+  direction TB
+  state One <<choice>>
+  state Two <<choice>>
+  state Three <<choice>>
+  One --> Two: clockwise
+  Two --> Three: clockwise
+  Three --> One: clockwise`)
+
+    expect(output.match(/◆/g)).toHaveLength(3)
+  })
+
+  test("routes dense horizontal transitions around unrelated states", () => {
+    const output = renderStateDiagram(`stateDiagram-v2
+  direction LR
+  A --> B: ab
+  A --> C: ac
+  A --> D: ad
+  B --> A: ba
+  B --> C: bc
+  B --> D: bd
+  C --> A: ca
+  C --> B: cb
+  C --> D: cd
+  D --> A: da
+  D --> B: db
+  D --> C: dc`)
+
+    for (const state of ["A", "B", "C", "D"]) expect(output.match(new RegExp(state, "g"))).toHaveLength(1)
+  })
+
+  test("routes parallel transitions around vertically offset states", () => {
+    const output = renderStateDiagram(`stateDiagram-v2
+  A --> B: first<br/>line two
+  A --> B: second<br/>another line
+  B --> A: return<br/>with details`)
+
+    expect(output).toContain(" A ")
+    expect(output).toContain("│ B │")
+    expect(output).toContain("first")
+    expect(output).toContain("second")
+    expect(output).toContain("return")
+  })
+
   test("keeps independent overlapping feedback labels and paths distinct", () => {
     const content = (direction: "LR" | "RL") => `stateDiagram-v2
   direction ${direction}
@@ -561,15 +631,46 @@ stateDiagram-v2
     })
     expect(output).toMatchInlineSnapshot(`
       "            ╭─ Authenticated ──────────────────╮
-                  │                                  │
-          login   │ ╭──────╮    open     ╭─────────╮ │  save
-      ●────────────▶│ Idle ├────────────▶│ Editing ├────────────▶◎
+                  │                                  │ save
+          login   │ ╭──────╮    open     ╭─────────╮ │ logout
+      ●───────────┼▶│ Idle ├────────────▶│ Editing ├─┼──────────▶◎
                   │ ╰──────╯             ╰─────────╯ │
                   │                                  │
                   ╰──────────────────────────────────╯"
     `)
   })
 
+  test("keeps nested composite entry and exit routes within the outer frame height", () => {
+    const output = renderStateDiagram(`stateDiagram-v2
+  state Session {
+    [*] --> Open
+    state Open {
+      [*] --> Clean
+      Clean --> Dirty: edit
+      Dirty --> Clean: save
+    }
+    note right of Open: document lifecycle
+    Open --> [*]: close
+  }
+  [*] --> Session
+    Session --> [*]`)
+    const lines = output.split("\n")
+    const outerFrameTop = lines.find((line) => line.includes("Session"))!
+    const frameLeft = outerFrameTop.indexOf("╭")
+    const frameRight = outerFrameTop.lastIndexOf("╮")
+    const outerFrameBottom = lines.findIndex((line) => line[frameLeft] === "╰" && line[frameRight] === "╯")
+    const startColumn = lines.find((line) => line.includes("●"))!.indexOf("●")
+    const endColumn = lines.find((line) => line.includes("◎"))!.indexOf("◎")
+
+    expect(outerFrameBottom).toBeGreaterThan(0)
+    expect(startColumn).toBeLessThan(frameLeft)
+    expect(endColumn).toBeGreaterThan(frameRight)
+    expect(lines.slice(outerFrameBottom + 1).every((line) => line.trim() === "")).toBe(true)
+    expect(output).toContain("Open")
+    expect(output).toContain("document lifecycle")
+    expect(output).toContain("close")
+  })
+
   test("renders notes attached to states", () => {
     const output = renderStateDiagram(`
 stateDiagram-v2
@@ -600,6 +701,91 @@ stateDiagram-v2
   state Decision <<choice>>
   Decision --> [*]`)
 
-    expect(output).toContain("╰─────────────┬\n")
+    expect(output).toContain("╰─────────────▼")
+    expect(output).toContain("◆────────────▶◎")
+  })
+
+  test("keeps vertical branch labels from overwriting state labels", () => {
+    const output = renderStateDiagram(`stateDiagram-v2
+  direction TB
+  state "Branch root" as Root
+  state "Upper branch" as Upper
+  state "Lower branch" as Lower
+  state "Merged branch" as Merge
+  Root --> Upper: branch-up
+  Root --> Lower: branch-down
+  Upper --> Merge: merge-up
+  Lower --> Merge: merge-down
+  Merge --> Root: branch-feedback`)
+
+    for (const text of [
+      "Branch root",
+      "Upper branch",
+      "Lower branch",
+      "Merged branch",
+      "branch-up",
+      "branch-down",
+      "merge-up",
+      "merge-down",
+      "branch-feedback",
+    ]) {
+      expect(output).toContain(text)
+    }
+  })
+
+  test("keeps lifecycle states intact around branches and feedback", () => {
+    const output = renderStateDiagram(`stateDiagram-v2
+  [*] --> Idle
+  Idle --> MailboxPending: enqueue + setAlarm
+  MailboxPending --> PromptSubmitted: drain mailbox
+  PromptSubmitted --> Polling: prompt admitted
+  Polling --> Polling: execution still active
+  Polling --> Completed: terminal log event
+  Polling --> Polling: retry after transient failure
+  Completed --> Idle: final Slack projection
+  Idle --> Expired: 30 days inactive
+  Expired --> [*]: delete SQLite state`)
+
+    for (const state of ["Idle", "MailboxPending", "PromptSubmitted", "Polling", "Completed", "Expired"]) {
+      expect(output.match(new RegExp(state, "g"))).toHaveLength(1)
+    }
+  })
+
+  test("keeps composite titles intact under reciprocal composite routes", () => {
+    const source = `stateDiagram-v2
+  direction LR
+  state FirstGroup {
+    [*] --> FirstInner
+    FirstInner --> [*]: first-out
+  }
+  state SecondGroup {
+    [*] --> SecondInner
+    SecondInner --> [*]: second-out
+  }
+  FirstGroup --> SecondGroup: group-next
+  SecondGroup --> FirstGroup: group-back`
+
+    for (const direction of ["LR", "TB"] as const) {
+      const lines = renderStateDiagram(source, { direction }).split("\n")
+      for (const title of ["FirstGroup", "SecondGroup"]) {
+        const top = lines.findIndex((line) => line.includes(title))
+        const left = lines[top]!.lastIndexOf("╭", lines[top]!.indexOf(title))
+        const right = lines[top]!.indexOf("╮", left)
+        const bottom = lines.findIndex((line, index) => index > top && line[left] === "╰" && line[right] === "╯")
+
+        expect(top).toBeGreaterThanOrEqual(0)
+        expect(left).toBeGreaterThanOrEqual(0)
+        expect(right).toBeGreaterThan(left)
+        expect(bottom).toBeGreaterThan(top)
+        expect(
+          lines.slice(top + 1, bottom).every((line) => "│├┤┼".includes(line[left]!) && "│├┤┼".includes(line[right]!)),
+        ).toBe(true)
+        expect(
+          lines[bottom]!.slice(left + 1, right)
+            .split("")
+            .every((char) => "─┬┴┼".includes(char)),
+        ).toBe(true)
+      }
+    }
   })
 })

packages/merman/src/state/drawing.ts

@@ -49,7 +49,9 @@ function translateTransitionPlans(
 function makeGrid(width: number, height: number): StateGrid {
   return new DiagramCanvas(width, height, {
     mergeCell: (existing, incoming): StateCell => {
-      const shouldMerge = existing.style === "transition" && incoming.style === "transition"
+      const existingIsTransition = existing.style === "transition" || existing.style?.startsWith("stateDepartureRamp")
+      const incomingIsTransition = incoming.style === "transition" || incoming.style?.startsWith("stateDepartureRamp")
+      const shouldMerge = incomingIsTransition && (existingIsTransition || existing.style === "composite")
       return {
         ...incoming,
         char: shouldMerge
@@ -198,7 +200,8 @@ function drawTransitionJunctionPlans(
 ): void {
   for (const plan of createStateTransitionJunctionPlans(diagram, bounds, renderPlans)) {
     const style = plan.kind === "choice" ? "choice" : "transition"
-    setCell(grid, plan.bounds.left, plan.bounds.top, diagramLineGlyph(plan.connections, "rounded"), style)
+    const char = plan.kind === "choice" ? "◆" : diagramLineGlyph(plan.connections, "rounded")
+    setCell(grid, plan.bounds.left, plan.bounds.top, char, style)
   }
 }
 

packages/merman/src/state/layout.ts

@@ -40,14 +40,6 @@ export interface StateDiagramLayoutOptions {
   minStateGap: number
 }
 
-function visualLength(value: string): number {
-  return diagramTextWidth(value)
-}
-
-function splitStateDiagramLines(value: string): string[] {
-  return splitDiagramLines(value)
-}
-
 function computeRanks(diagram: StateDiagram): Map<string, number> {
   const ranks = new Map<string, number>()
   const outgoing = new Map<string, string[]>()
@@ -88,8 +80,11 @@ function outgoingTransitions(diagram: StateDiagram): Map<string, StateDiagramTra
   return outgoing
 }
 
-function reaches(diagram: StateDiagram, from: string, target: string): boolean {
-  const outgoing = outgoingTransitions(diagram)
+function reaches(
+  outgoing: ReadonlyMap<string, readonly StateDiagramTransition[]>,
+  from: string,
+  target: string,
+): boolean {
   const visited = new Set<string>()
   const stack = [from]
   while (stack.length > 0) {
@@ -104,6 +99,7 @@ function reaches(diagram: StateDiagram, from: string, target: string): boolean {
 
 function computeMainPath(diagram: StateDiagram): string[] {
   const outgoing = outgoingTransitions(diagram)
+  const statesById = new Map(diagram.states.map((state) => [state.id, state]))
   const start = diagram.states.find((state) => state.kind === "start")?.id ?? diagram.states[0]?.id
   if (!start) return []
 
@@ -114,9 +110,14 @@ function computeMainPath(diagram: StateDiagram): string[] {
     const candidates = (outgoing.get(current) ?? []).filter((transition) => !visited.has(transition.to))
     if (candidates.length === 0) break
     const next =
-      candidates.find((transition) => diagram.states.find((state) => state.id === transition.to)?.kind === "end") ??
-      candidates.find((transition) => !reaches(diagram, transition.to, current)) ??
-      candidates.find((transition) => !hasReverseTransition(diagram, transition))
+      candidates.find((transition) => statesById.get(transition.to)?.kind === "end") ??
+      candidates.find((transition) => !reaches(outgoing, transition.to, current)) ??
+      candidates.find((transition) => !hasReverseTransition(diagram, transition)) ??
+      candidates.find((transition) => {
+        const fromParent = statesById.get(current)?.parentId
+        const toParent = statesById.get(transition.to)?.parentId
+        return Boolean(fromParent && toParent && fromParent !== toParent)
+      })
     if (!next) break
     path.push(next.to)
     visited.add(next.to)
@@ -132,7 +133,7 @@ function stateSize(state: StateDiagramState): { width: number; height: number; l
 }
 
 function noteLines(note: StateDiagramNote): string[] {
-  const lines = note.lines.flatMap(splitStateDiagramLines).map((line) => line.trim())
+  const lines = note.lines.flatMap(splitDiagramLines).map((line) => line.trim())
   return lines.length > 0 ? lines : [""]
 }
 
@@ -202,7 +203,7 @@ function addCompositeBounds(diagram: StateDiagram, layout: StateDiagramLayout):
     const top = Math.min(...childBounds.map((bound) => bound.top)) - 2
     const right = Math.max(...childBounds.map((bound) => bound.left + bound.width)) + 2
     const bottom = Math.max(...childBounds.map((bound) => bound.top + bound.height)) + 2
-    const width = Math.max(right - left, visualLength(composite.label) + 5)
+    const width = Math.max(right - left, diagramTextWidth(composite.label) + 5)
     const bound = {
       id: composite.id,
       left,
@@ -349,7 +350,7 @@ function expandCompositeBoundsForNotes(diagram: StateDiagram, layout: StateDiagr
 
     bound.left = left
     bound.top = top
-    bound.width = Math.max(right - left, visualLength(composite.label) + 5)
+    bound.width = Math.max(right - left, diagramTextWidth(composite.label) + 5)
     bound.height = bottom - top
     bound.centerX = bound.left + Math.floor(bound.width / 2)
     bound.centerY = bound.top + Math.floor(bound.height / 2)
@@ -461,7 +462,8 @@ export function createStateDiagramLayout(
       x += size.width + options.minStateGap + 8
     }
     const labelRows = states.reduce((rows, state) => Math.max(rows, outgoingLabelRows.get(state.id) ?? 0), 0)
-    y += rowHeight + Math.max(4, labelRows + 3)
+    const pseudoStateApproachClearance = states.some((state) => state.kind === "choice") ? 2 : 0
+    y += rowHeight + Math.max(4, labelRows + 3) + pseudoStateApproachClearance
   }
 
   return finalizeLayout(diagram, emptyLayout(bounds, sizes))
@@ -499,7 +501,10 @@ function createHorizontalLayout(diagram: StateDiagram, options: StateDiagramLayo
     const adjacentLabelWidth = diagram.transitions
       .filter((transition) => transition.from === id && transition.to === nextId)
       .reduce((width, transition) => Math.max(width, measureStateTransitionLabel(transition.label).width), 0)
-    x += size.width + Math.max(defaultGap, adjacentLabelWidth + 2)
+    const crossesCompositeBoundary = Boolean(
+      nextId && statesById.get(id)?.parentId !== statesById.get(nextId)?.parentId,
+    )
+    x += size.width + Math.max(defaultGap, adjacentLabelWidth + (crossesCompositeBoundary ? 6 : 2))
   }
 
   const branchesByParent = new Map<string, string[]>()
@@ -550,12 +555,25 @@ function createHorizontalLayout(diagram: StateDiagram, options: StateDiagramLayo
   }
 
   const ranks = computeRanks(diagram)
-  const fallbackStates = diagram.states.filter((state) => !bounds.has(state.id))
+  const fallbackStates = diagram.states
+    .filter((state) => !bounds.has(state.id))
+    .sort((left, right) => (ranks.get(left.id) ?? 0) - (ranks.get(right.id) ?? 0))
   for (const state of fallbackStates) {
     const size = sizes.get(state.id)!
-    const rank = ranks.get(state.id) ?? bounds.size
     const top = baselineY + 5
-    const left = rank * (size.width + defaultGap)
+    const rank = ranks.get(state.id) ?? bounds.size
+    let left = rank * (size.width + defaultGap)
+    while (true) {
+      const collision = [...bounds.values()].find(
+        (bound) =>
+          left < bound.left + bound.width + defaultGap &&
+          left + size.width + defaultGap > bound.left &&
+          top < bound.top + bound.height &&
+          top + size.height > bound.top,
+      )
+      if (!collision) break
+      left = collision.left + collision.width + defaultGap
+    }
     bounds.set(state.id, {
       id: state.id,
       left,

packages/merman/src/state/parser.ts

@@ -1,4 +1,4 @@
-import { firstMeaningfulMermaidLine, numberedMermaidLines } from "../core/mermaid.js"
+import { decodeMermaidText, firstMeaningfulMermaidLine, numberedMermaidLines } from "../core/mermaid.js"
 import { splitDiagramLines } from "../core/text-lines.js"
 import { MermaidSyntaxError } from "../diagnostics.js"
 import { normalizeStateDiagramEndpoint, stateDiagramEndMarkerId, stateDiagramStartMarkerId } from "./endpoint.js"
@@ -96,7 +96,7 @@ export function parseMermaidStateDiagram(content: string): StateDiagram {
         notes.push({
           target: pendingNote.target,
           position: pendingNote.position,
-          lines: pendingNote.lines,
+          lines: pendingNote.lines.map(decodeMermaidText),
         })
         pendingNote = undefined
       } else if (line || pendingNote.lines.length > 0) {
@@ -119,6 +119,9 @@ export function parseMermaidStateDiagram(content: string): StateDiagram {
 
     const directionMatch = line.match(DIRECTION_RE)
     if (directionMatch) {
+      if (parentStack.length > 0) {
+        throw new MermaidSyntaxError("state", source.lineNumber, line, "Composite-local direction is not supported")
+      }
       direction = normalizeDirection(directionMatch[1])
       continue
     }
@@ -128,7 +131,7 @@ export function parseMermaidStateDiagram(content: string): StateDiagram {
       notes.push({
         position: inlineNoteMatch[1]!.toLowerCase() as "left" | "right",
         target: inlineNoteMatch[2]!,
-        lines: splitDiagramLines(inlineNoteMatch[3]!.trim()),
+        lines: splitDiagramLines(decodeMermaidText(inlineNoteMatch[3]!.trim())),
       })
       continue
     }
@@ -150,7 +153,7 @@ export function parseMermaidStateDiagram(content: string): StateDiagram {
       const id = compositeMatch[2]!
       composites.push({
         id,
-        label: compositeMatch[1] ?? id,
+        label: decodeMermaidText(compositeMatch[1] ?? id),
         ...(parentId ? { parentId } : {}),
       })
       parentStack.push({ id, lineNumber: source.lineNumber, sourceLine: line })
@@ -159,13 +162,13 @@ export function parseMermaidStateDiagram(content: string): StateDiagram {
 
     const stateMatch = line.match(STATE_RE)
     if (stateMatch) {
-      ensureState(states, stateMatch[2]!, stateMatch[1]!, "state", parentId)
+      ensureState(states, stateMatch[2]!, decodeMermaidText(stateMatch[1]!), "state", parentId)
       continue
     }
 
     const choiceMatch = line.match(CHOICE_STATE_RE)
     if (choiceMatch) {
-      ensureState(states, choiceMatch[1]!, "┼", "choice", parentId)
+      ensureState(states, choiceMatch[1]!, "", "choice", parentId)
       continue
     }
 
@@ -177,7 +180,7 @@ export function parseMermaidStateDiagram(content: string): StateDiagram {
       const to = normalizeStateDiagramEndpoint(rawTo, "to", parentId)
       ensureState(states, from, rawFrom === "[*]" ? "●" : from, rawFrom === "[*]" ? "start" : "state", parentId)
       ensureState(states, to, rawTo === "[*]" ? "◎" : to, rawTo === "[*]" ? "end" : "state", parentId)
-      transitions.push({ from, to, label: transitionMatch[3]?.trim() ?? "" })
+      transitions.push({ from, to, label: decodeMermaidText(transitionMatch[3]?.trim() ?? "") })
       continue
     }
 

packages/merman/src/state/routing.test.ts

@@ -1,11 +1,14 @@
 import { describe, expect, test } from "bun:test"
 import type { StateDiagramBoxBounds } from "./layout.js"
+import { createStateDiagramLayout } from "./layout.js"
+import { parseMermaidStateDiagram } from "./parser.js"
 import {
   createStateTransitionJunctionPlans,
   createStateTransitionRenderPlans,
   createStateTransitionRoutePlans,
 } from "./routing.js"
-import { prepareVisibleStateDiagram, type StateVisibleDiagram } from "./visible-model.js"
+import type { StateVisibleDiagram } from "./visible-model.js"
+import { prepareVisibleStateDiagram } from "./visible-model.js"
 
 function bounds(id: string, centerX: number, centerY: number): StateDiagramBoxBounds {
   return { id, left: centerX - 2, top: centerY - 1, width: 5, height: 3, centerX, centerY }
@@ -207,6 +210,39 @@ describe("createStateTransitionRenderPlans", () => {
       [11, 4],
     ])
   })
+
+  test("keeps vertical branch routes out of unrelated state bounds", () => {
+    const diagram = prepareVisibleStateDiagram(
+      parseMermaidStateDiagram(`stateDiagram-v2
+  direction TB
+  state "Branch root" as Root
+  state "Upper branch" as Upper
+  state "Lower branch" as Lower
+  state "Merged branch" as Merge
+  Root --> Upper: branch-up
+  Root --> Lower: branch-down
+  Upper --> Merge: merge-up
+  Lower --> Merge: merge-down
+  Merge --> Root: branch-feedback`),
+    )
+    const layout = createStateDiagramLayout(diagram, { minStateGap: 4 })
+    const plans = createStateTransitionRenderPlans(diagram, layout.bounds, 30)
+
+    for (const plan of plans) {
+      const unrelated = diagram.states
+        .filter((state) => state.id !== plan.route.transition.from && state.id !== plan.route.transition.to)
+        .map((state) => layout.bounds.get(state.id)!)
+      expect(
+        plan.path.some(([x, y]) =>
+          unrelated.some(
+            (bound) =>
+              x >= bound.left && x < bound.left + bound.width && y >= bound.top && y < bound.top + bound.height,
+          ),
+        ),
+        `${plan.route.transition.from} -> ${plan.route.transition.to}`,
+      ).toBe(false)
+    }
+  })
 })
 
 describe("createStateTransitionJunctionPlans", () => {

packages/merman/src/state/routing.ts

@@ -1,5 +1,6 @@
 import { BorderChars } from "@opentui/core"
 import type { DiagramDirection } from "../core/geometry.js"
+import { SpatialIndex, spatialPathClaim, spatialRectClaim } from "../core/spatial.js"
 import { diagramTextWidth, splitDiagramLines } from "../core/text.js"
 import type { StateDiagramBoxBounds as BoxBounds } from "./layout.js"
 import type { StateDiagram, StateDiagramState, StateDiagramTransition } from "./types.js"
@@ -10,14 +11,15 @@ interface StateTransitionRoutePlanBase {
   from: BoxBounds
   to: BoxBounds
   targetIsChoice: boolean
+  targetIsHiddenMarker: boolean
 }
 
 export type StateTransitionRoutePlan =
   | (StateTransitionRoutePlanBase & { kind: "self" })
   | (StateTransitionRoutePlanBase & { kind: "horizontal-forward"; leftToRight: boolean })
-  | (StateTransitionRoutePlanBase & { kind: "bottom-feedback"; railY: number })
+  | (StateTransitionRoutePlanBase & { kind: "bottom-feedback"; railY: number; approachX: number })
   | (StateTransitionRoutePlanBase & { kind: "top-feedback"; railY: number })
-  | (StateTransitionRoutePlanBase & { kind: "bottom-parallel"; railY: number })
+  | (StateTransitionRoutePlanBase & { kind: "bottom-parallel"; railY: number; approachX: number })
   | (StateTransitionRoutePlanBase & { kind: "vertical-elbow"; hasReverse: boolean; offsetConnector: boolean })
   | (StateTransitionRoutePlanBase & { kind: "side-parallel"; railX: number })
   | (StateTransitionRoutePlanBase & { kind: "vertical" })
@@ -192,6 +194,93 @@ function hasOpposingTopConnector(
   })
 }
 
+function verticalCorridorCrossesUnrelatedState(
+  diagram: StateVisibleDiagram,
+  transition: StateVisibleTransition,
+  from: BoxBounds,
+  to: BoxBounds,
+  bounds: ReadonlyMap<string, BoxBounds>,
+): boolean {
+  const top = Math.min(from.top + from.height, to.top + to.height)
+  const bottom = Math.max(from.top - 1, to.top - 1)
+  return diagram.states.some((state) => {
+    if (state.id === transition.from || state.id === transition.to || isHiddenCompositeMarker(state)) return false
+    const bound = bounds.get(state.id)
+    return Boolean(
+      bound &&
+      from.centerX >= bound.left &&
+      from.centerX < bound.left + bound.width &&
+      top < bound.top + bound.height &&
+      bottom >= bound.top,
+    )
+  })
+}
+
+function horizontalCorridorCrossesUnrelatedState(
+  diagram: StateVisibleDiagram,
+  transition: StateVisibleTransition,
+  from: BoxBounds,
+  to: BoxBounds,
+  bounds: ReadonlyMap<string, BoxBounds>,
+): boolean {
+  const leftToRight = from.centerX <= to.centerX
+  const startX = leftToRight ? from.left + from.width : from.left - 1
+  const endX = leftToRight ? to.left - 1 : to.left + to.width
+  const space = SpatialIndex.empty().add(
+    ...diagram.states.flatMap((state) => {
+      if (state.id === transition.from || state.id === transition.to || isHiddenCompositeMarker(state)) return []
+      const bound = bounds.get(state.id)
+      return bound ? [spatialRectClaim(`state:${state.id}`, `state:${state.id}`, "body", bound)] : []
+    }),
+  )
+  const corridor = spatialPathClaim(
+    `corridor:${transition.from}:${transition.to}`,
+    `transition:${transition.from}:${transition.to}`,
+    "route",
+    [
+      { x: startX, y: from.centerY },
+      { x: endX, y: from.centerY },
+    ],
+  )
+  return !space.isFree(corridor)
+}
+
+function bottomApproachX(
+  diagram: StateVisibleDiagram,
+  transition: StateVisibleTransition,
+  from: BoxBounds,
+  to: BoxBounds,
+  bounds: ReadonlyMap<string, BoxBounds>,
+  railY: number,
+): number {
+  const targetX = to.width > 1 ? (from.centerX > to.centerX ? to.left + 1 : to.left + to.width - 2) : to.centerX
+  const targetBottomY = to.top + to.height
+  const top = Math.min(targetBottomY, railY)
+  const bottom = Math.max(targetBottomY, railY)
+  const isClear = (x: number): boolean =>
+    !diagram.states.some((state) => {
+      if (state.id === transition.from || state.id === transition.to || isHiddenCompositeMarker(state)) return false
+      const bound = bounds.get(state.id)
+      return Boolean(
+        bound &&
+        x >= bound.left &&
+        x < bound.left + bound.width &&
+        top < bound.top + bound.height &&
+        bottom >= bound.top,
+      )
+    })
+
+  if (isClear(targetX)) return targetX
+  const maxX = Math.max(targetX, ...[...bounds.values()].map((bound) => bound.left + bound.width)) + 1
+  for (let distance = 1; distance <= maxX; distance++) {
+    const right = targetX + distance
+    if (isClear(right)) return right
+    const left = targetX - distance
+    if (left >= 0 && isClear(left)) return left
+  }
+  return targetX
+}
+
 export function createStateTransitionRoutePlans(
   diagram: StateVisibleDiagram,
   bounds: ReadonlyMap<string, BoxBounds>,
@@ -200,16 +289,29 @@ export function createStateTransitionRoutePlans(
 ): StateTransitionRoutePlan[] {
   const statesById = new Map(diagram.states.map((state) => [state.id, state]))
   const endpointOccurrences = new Map<string, number>()
-  const maxLabelWidth = Math.max(
-    0,
-    ...diagram.transitions.map((transition) => measureStateTransitionLabel(transition.label).width),
-  )
   const parallelLaneGap = Math.max(
     3,
     ...diagram.transitions.map((transition) => measureStateTransitionLabel(transition.label).height + 2),
   )
-  const sideLaneX = Math.max(0, ...[...bounds.values()].map((bound) => bound.left + bound.width)) + maxLabelWidth + 3
+  let nextSideRailX = Math.max(0, ...[...bounds.values()].map((bound) => bound.left + bound.width)) + 3
   const feedbackAllocations = createFeedbackAllocations(diagram, bounds, feedbackLaneY, parallelLaneGap, feedbackTopY)
+  let nextBottomRailY =
+    Math.max(
+      feedbackLaneY - parallelLaneGap,
+      ...[...feedbackAllocations.values()]
+        .filter((allocation) => allocation.side === "bottom")
+        .map((allocation) => allocation.railY),
+    ) + parallelLaneGap
+  const allocateSideRail = (label: string): number => {
+    const railX = nextSideRailX
+    nextSideRailX += Math.max(3, measureStateTransitionLabel(label).width + 2)
+    return railX
+  }
+  const allocateBottomRail = (): number => {
+    const railY = nextBottomRailY
+    nextBottomRailY += parallelLaneGap
+    return railY
+  }
 
   return diagram.transitions.flatMap((transition): StateTransitionRoutePlan[] => {
     const from = bounds.get(transition.from)
@@ -217,8 +319,9 @@ export function createStateTransitionRoutePlans(
     if (!from || !to) return []
 
     const targetState = statesById.get(transition.to)
-    const targetIsChoice = targetState?.kind === "choice" || isHiddenCompositeMarker(targetState)
-    const base = { transition, from, to, targetIsChoice }
+    const targetIsChoice = targetState?.kind === "choice"
+    const targetIsHiddenMarker = isHiddenCompositeMarker(targetState)
+    const base = { transition, from, to, targetIsChoice, targetIsHiddenMarker }
     if (transition.from === transition.to) return [{ ...base, kind: "self" }]
     const endpointKey = `${transition.from}\u0000${transition.to}`
     const parallelIndex = endpointOccurrences.get(endpointKey) ?? 0
@@ -227,30 +330,80 @@ export function createStateTransitionRoutePlans(
       (diagram.direction === "LR" || diagram.direction === "RL") && isStateHorizontalFeedback(diagram, from, to)
     const feedbackAllocation = feedbackAllocations.get(transition)
     if (feedbackAllocation) {
+      if (feedbackAllocation.side === "bottom") {
+        return [
+          {
+            ...base,
+            kind: "bottom-feedback",
+            railY: feedbackAllocation.railY,
+            approachX: bottomApproachX(diagram, transition, from, to, bounds, feedbackAllocation.railY),
+          },
+        ]
+      }
       return [
         {
           ...base,
-          kind: feedbackAllocation.side === "bottom" ? "bottom-feedback" : "top-feedback",
+          kind: "top-feedback",
           railY: feedbackAllocation.railY,
         },
       ]
     }
     if (parallelIndex > 0) {
-      if (diagram.direction === "LR" || diagram.direction === "RL") {
+      if ((diagram.direction === "LR" || diagram.direction === "RL") && from.centerY === to.centerY) {
+        const railY = allocateBottomRail()
         return [
           {
             ...base,
             kind: "bottom-parallel",
-            railY: feedbackLaneY + (parallelIndex - 1) * parallelLaneGap,
+            railY,
+            approachX: bottomApproachX(diagram, transition, from, to, bounds, railY),
           },
         ]
       }
-      return [{ ...base, kind: "side-parallel", railX: sideLaneX + (parallelIndex - 1) * parallelLaneGap }]
+      return [{ ...base, kind: "side-parallel", railX: allocateSideRail(transition.label) }]
+    }
+    if (diagram.direction !== "LR" && diagram.direction !== "RL") {
+      const fromParent = statesById.get(transition.from)?.parentId
+      const toParent = statesById.get(transition.to)?.parentId
+      if (fromParent && toParent && fromParent !== toParent) {
+        return [{ ...base, kind: "side-parallel", railX: allocateSideRail(transition.label) }]
+      }
+      if (verticalCorridorCrossesUnrelatedState(diagram, transition, from, to, bounds)) {
+        return [{ ...base, kind: "side-parallel", railX: allocateSideRail(transition.label) }]
+      }
+      if (from.centerY > to.centerY) {
+        return [{ ...base, kind: "side-parallel", railX: allocateSideRail(transition.label) }]
+      }
+      if (from.centerY === to.centerY) {
+        if (hasReverseTransition(diagram, transition) && from.centerX > to.centerX) {
+          const railY = allocateBottomRail()
+          return [
+            {
+              ...base,
+              kind: "bottom-parallel",
+              railY,
+              approachX: bottomApproachX(diagram, transition, from, to, bounds, railY),
+            },
+          ]
+        }
+        return [{ ...base, kind: "horizontal-forward", leftToRight: from.centerX <= to.centerX }]
+      }
+      if (from.centerX !== to.centerX) {
+        return [{ ...base, kind: "vertical-elbow", hasReverse: false, offsetConnector: false }]
+      }
+      return [{ ...base, kind: "vertical" }]
     }
-    if (diagram.direction !== "LR" && diagram.direction !== "RL") return [{ ...base, kind: "vertical" }]
 
     if (from.centerY !== to.centerY) {
-      if (from.centerY > to.centerY && feedback) return [{ ...base, kind: "bottom-feedback", railY: feedbackLaneY }]
+      if (from.centerY > to.centerY && feedback)
+        return [
+          {
+            ...base,
+            kind: "bottom-feedback",
+            railY: feedbackLaneY,
+            approachX: bottomApproachX(diagram, transition, from, to, bounds, feedbackLaneY),
+          },
+        ]
       const hasReverse = hasReverseTransition(diagram, transition)
       return [
         {
@@ -261,7 +414,26 @@ export function createStateTransitionRoutePlans(
         },
       ]
     }
-    if (feedback) return [{ ...base, kind: "bottom-feedback", railY: feedbackLaneY }]
+    if (feedback)
+      return [
+        {
+          ...base,
+          kind: "bottom-feedback",
+          railY: feedbackLaneY,
+          approachX: bottomApproachX(diagram, transition, from, to, bounds, feedbackLaneY),
+        },
+      ]
+    if (horizontalCorridorCrossesUnrelatedState(diagram, transition, from, to, bounds)) {
+      const railY = allocateBottomRail()
+      return [
+        {
+          ...base,
+          kind: "bottom-parallel",
+          railY,
+          approachX: bottomApproachX(diagram, transition, from, to, bounds, railY),
+        },
+      ]
+    }
     return [{ ...base, kind: "horizontal-forward", leftToRight: from.centerX <= to.centerX }]
   })
 }
@@ -333,7 +505,7 @@ function addTopDeparture(builder: StateTransitionRenderBuilder, bounds: BoxBound
 }
 
 function addHorizontalForward(builder: StateTransitionRenderBuilder): void {
-  const { from, to, targetIsChoice, leftToRight, transition } = builder.route as Extract<
+  const { from, to, targetIsChoice, targetIsHiddenMarker, leftToRight, transition } = builder.route as Extract<
     StateTransitionRoutePlan,
     { kind: "horizontal-forward" }
   >
@@ -343,9 +515,12 @@ function addHorizontalForward(builder: StateTransitionRenderBuilder): void {
   const step = leftToRight ? 1 : -1
   const startX = leftToRight ? from.left + from.width : from.left - 1
   const endX = leftToRight ? to.left - 1 : to.left + to.width
-  addHorizontalLine(builder, startX, targetIsChoice ? endX : endX - step, y, step)
-  if (targetIsChoice) addPathPoint(builder, to.left, y)
-  else addCell(builder, { x: endX, y, arrowDirection: leftToRight ? "right" : "left" })
+  addHorizontalLine(builder, startX, endX - step, y, step)
+  addCell(
+    builder,
+    targetIsHiddenMarker ? { x: endX, y, char: "─" } : { x: endX, y, arrowDirection: leftToRight ? "right" : "left" },
+  )
+  if (targetIsChoice || targetIsHiddenMarker) addPathPoint(builder, to.left, y)
   if (!transition.label) return
   const metrics = measureStateTransitionLabel(transition.label)
   const labelX = Math.min(startX, endX) + Math.max(1, Math.floor((Math.abs(endX - startX) - metrics.width) / 2))
@@ -378,14 +553,14 @@ function outsideTopY(bounds: BoxBounds): number {
 }
 
 function addBottomLaneTransition(builder: StateTransitionRenderBuilder): void {
-  const { from, to, targetIsChoice, transition, railY } = builder.route as Extract<
+  const { from, to, targetIsChoice, targetIsHiddenMarker, transition, railY, approachX } = builder.route as Extract<
     StateTransitionRoutePlan,
     { kind: "bottom-feedback" | "bottom-parallel" }
   >
   const sourceX = from.centerX
   const targetX = to.width > 1 ? (sourceX > to.centerX ? to.left + 1 : to.left + to.width - 2) : to.centerX
   const targetRailCutsSource = targetX >= from.left && targetX <= from.left + from.width - 1
-  const railTargetX = targetRailCutsSource ? Math.max(from.left + from.width, to.left + to.width) + 2 : targetX
+  const railTargetX = targetRailCutsSource ? Math.max(from.left + from.width, to.left + to.width) + 2 : approachX
   const sourceBottomY = outsideBottomY(from)
   const targetBottomY = outsideBottomY(to)
   addBottomDeparture(builder, from, sourceX)
@@ -406,8 +581,13 @@ function addBottomLaneTransition(builder: StateTransitionRenderBuilder): void {
       addCell(builder, { x, y: targetBottomY, char: "─" })
     }
   }
-  addCell(builder, { x: targetX, y: targetBottomY, ...(targetIsChoice ? { char: "│" } : { arrowDirection: "up" }) })
-  if (targetIsChoice) addPathPoint(builder, to.left, to.top)
+  addCell(
+    builder,
+    targetIsHiddenMarker
+      ? { x: targetX, y: targetBottomY, char: "│" }
+      : { x: targetX, y: targetBottomY, arrowDirection: "up" },
+  )
+  if (targetIsChoice || targetIsHiddenMarker) addPathPoint(builder, to.left, to.top)
   if (!transition.label) return
   const metrics = measureStateTransitionLabel(transition.label)
   const horizontalRoom = Math.abs(sourceX - railTargetX) - 2
@@ -419,7 +599,7 @@ function addBottomLaneTransition(builder: StateTransitionRenderBuilder): void {
 }
 
 function addTopFeedbackTransition(builder: StateTransitionRenderBuilder): void {
-  const { from, to, targetIsChoice, transition, railY } = builder.route as Extract<
+  const { from, to, targetIsChoice, targetIsHiddenMarker, transition, railY } = builder.route as Extract<
     StateTransitionRoutePlan,
     { kind: "top-feedback" }
   >
@@ -437,8 +617,13 @@ function addTopFeedbackTransition(builder: StateTransitionRenderBuilder): void {
   }
   addCell(builder, { x: targetX, y: railY, char: sourceX > targetX ? "╭" : "╮" })
   for (let y = railY + 1; y < targetTopY; y++) addCell(builder, { x: targetX, y, char: "│" })
-  addCell(builder, { x: targetX, y: targetTopY, ...(targetIsChoice ? { char: "│" } : { arrowDirection: "down" }) })
-  if (targetIsChoice) addPathPoint(builder, to.left, to.top)
+  addCell(
+    builder,
+    targetIsHiddenMarker
+      ? { x: targetX, y: targetTopY, char: "│" }
+      : { x: targetX, y: targetTopY, arrowDirection: "down" },
+  )
+  if (targetIsChoice || targetIsHiddenMarker) addPathPoint(builder, to.left, to.top)
   if (!transition.label) return
   const metrics = measureStateTransitionLabel(transition.label)
   const horizontalRoom = Math.abs(sourceX - targetX) - 2
@@ -450,7 +635,7 @@ function addTopFeedbackTransition(builder: StateTransitionRenderBuilder): void {
 }
 
 function addSideParallelTransition(builder: StateTransitionRenderBuilder): void {
-  const { from, to, targetIsChoice, transition, railX } = builder.route as Extract<
+  const { from, to, targetIsChoice, targetIsHiddenMarker, transition, railX } = builder.route as Extract<
     StateTransitionRoutePlan,
     { kind: "side-parallel" }
   >
@@ -465,9 +650,16 @@ function addSideParallelTransition(builder: StateTransitionRenderBuilder): void
   for (let y = startY + verticalStep; y !== endY; y += verticalStep) addCell(builder, { x: railX, y, char: "│" })
   addCell(builder, { x: railX, y: endY, char: verticalStep === 1 ? "╯" : "╮" })
   for (let x = railX - 1; x > endX; x--) addCell(builder, { x, y: endY, char: "─" })
-  addCell(builder, { x: endX, y: endY, ...(targetIsChoice ? { char: "─" } : { arrowDirection: "left" }) })
-  if (targetIsChoice) addPathPoint(builder, to.left, to.top)
-  if (transition.label) addLabel(builder, railX + 2, Math.min(startY, endY) + 1, transition.label)
+  addCell(
+    builder,
+    targetIsHiddenMarker ? { x: endX, y: endY, char: "─" } : { x: endX, y: endY, arrowDirection: "left" },
+  )
+  if (targetIsChoice || targetIsHiddenMarker) addPathPoint(builder, to.left, to.top)
+  if (transition.label) {
+    const metrics = measureStateTransitionLabel(transition.label)
+    const labelY = Math.max(0, Math.floor((startY + endY - metrics.height + 1) / 2))
+    addLabel(builder, railX + 2, labelY, transition.label)
+  }
 }
 
 function innerConnectorX(bounds: BoxBounds, preferredX: number): number {
@@ -476,10 +668,8 @@ function innerConnectorX(bounds: BoxBounds, preferredX: number): number {
 }
 
 function addVerticalElbowTransition(builder: StateTransitionRenderBuilder): void {
-  const { from, to, transition, targetIsChoice, hasReverse, offsetConnector } = builder.route as Extract<
-    StateTransitionRoutePlan,
-    { kind: "vertical-elbow" }
-  >
+  const { from, to, transition, targetIsChoice, targetIsHiddenMarker, hasReverse, offsetConnector } =
+    builder.route as Extract<StateTransitionRoutePlan, { kind: "vertical-elbow" }>
   const topToBottom = from.centerY < to.centerY
   const offset = offsetConnector ? (topToBottom ? -2 : 2) : 0
   const startX = innerConnectorX(from, from.centerX + offset)
@@ -513,13 +703,19 @@ function addVerticalElbowTransition(builder: StateTransitionRenderBuilder): void
       }
     }
   }
-  const targetChar = targetIsChoice ? (hasTargetApproach || startX === endX ? "│" : topToBottom ? "┬" : "┴") : undefined
+  const targetChar = targetIsHiddenMarker
+    ? hasTargetApproach || startX === endX
+      ? "│"
+      : topToBottom
+        ? "┬"
+        : "┴"
+    : undefined
   addCell(builder, {
     x: endX,
     y: endY,
     ...(targetChar ? { char: targetChar } : { arrowDirection: topToBottom ? "down" : "up" }),
   })
-  if (targetIsChoice) addPathPoint(builder, to.left, to.top)
+  if (targetIsChoice || targetIsHiddenMarker) addPathPoint(builder, to.left, to.top)
   if (!transition.label) return
   const metrics = measureStateTransitionLabel(transition.label)
   if (topToBottom) {
@@ -543,7 +739,7 @@ function addVerticalElbowTransition(builder: StateTransitionRenderBuilder): void
 }
 
 function addVerticalTransition(builder: StateTransitionRenderBuilder): void {
-  const { from, to, transition, targetIsChoice } = builder.route
+  const { from, to, transition, targetIsChoice, targetIsHiddenMarker } = builder.route
   const topToBottom = from.centerY <= to.centerY
   const x = from.centerX
   const startY = topToBottom ? from.top + from.height : from.top - 1
@@ -555,9 +751,9 @@ function addVerticalTransition(builder: StateTransitionRenderBuilder): void {
   addCell(builder, {
     x,
     y: endY,
-    ...(targetIsChoice ? { char: "│" } : { arrowDirection: topToBottom ? "down" : "up" }),
+    ...(targetIsHiddenMarker ? { char: "│" } : { arrowDirection: topToBottom ? "down" : "up" }),
   })
-  if (targetIsChoice) addPathPoint(builder, to.left, to.top)
+  if (targetIsChoice || targetIsHiddenMarker) addPathPoint(builder, to.left, to.top)
   if (transition.label) addLabel(builder, x + 2, Math.min(startY, endY) + 1, transition.label)
 }
 
@@ -590,69 +786,47 @@ function createStateTransitionRenderPlan(route: StateTransitionRoutePlan): State
   return builder
 }
 
-interface StateTransitionLabelRect {
-  left: number
-  top: number
-  width: number
-  height: number
-}
-
-function labelRect(label: StateTransitionRenderLabel, width: number): StateTransitionLabelRect {
-  return { left: label.x, top: label.y, width, height: label.lines.length }
-}
-
-function rectsOverlap(left: StateTransitionLabelRect, right: StateTransitionLabelRect): boolean {
-  return (
-    left.left < right.left + right.width &&
-    left.left + left.width > right.left &&
-    left.top < right.top + right.height &&
-    left.top + left.height > right.top
-  )
-}
-
 function placeStateTransitionLabels(
   plans: readonly StateTransitionRenderPlan[],
   diagram: StateVisibleDiagram,
   bounds: ReadonlyMap<string, BoxBounds>,
 ): StateTransitionRenderPlan[] {
-  const routeCells = new Set(plans.flatMap((plan) => plan.cells.map((cell) => `${cell.x}:${cell.y}`)))
-  const placedLabels: StateTransitionLabelRect[] = []
-  const stateRects = diagram.states.flatMap((state) => {
-    const bound = bounds.get(state.id)
-    return bound && !isHiddenCompositeMarker(state)
-      ? [{ left: bound.left, top: bound.top, width: bound.width, height: bound.height }]
-      : []
-  })
+  let space = SpatialIndex.empty().add(
+    ...diagram.states.flatMap((state) => {
+      const bound = bounds.get(state.id)
+      return bound && !isHiddenCompositeMarker(state)
+        ? [spatialRectClaim(`state:${state.id}`, `state:${state.id}`, "body", bound)]
+        : []
+    }),
+    ...plans.map((plan, index) =>
+      spatialPathClaim(
+        `route:${index}`,
+        `route:${index}`,
+        "route",
+        plan.path.map(([x, y]) => ({ x, y })),
+      ),
+    ),
+  )
 
-  return plans.map((plan) => {
+  return plans.map((plan, planIndex) => {
     if (!plan.label) return plan
     const width = Math.max(...plan.label.lines.map(diagramTextWidth))
-    if (plan.label.lines.length === 1) {
-      placedLabels.push(labelRect(plan.label, width))
-      return plan
-    }
-    const statePadding = 1
+    const statePadding = plan.label.lines.length === 1 ? 0 : 1
+    const labelClaim = (x: number, y: number) =>
+      spatialRectClaim(`label:${planIndex}`, `label:${planIndex}`, "label", {
+        left: x,
+        top: y,
+        width,
+        height: plan.label!.lines.length,
+      })
     const isClear = (x: number, y: number): boolean => {
       if (x < 0 || y < 0) return false
-      const rect = labelRect({ ...plan.label!, x, y }, width)
-      if (
-        stateRects.some((state) =>
-          rectsOverlap(rect, {
-            left: state.left - statePadding,
-            top: state.top - statePadding,
-            width: state.width + statePadding * 2,
-            height: state.height + statePadding * 2,
-          }),
-        )
-      )
-        return false
-      if (placedLabels.some((label) => rectsOverlap(rect, label))) return false
-      for (let row = rect.top; row < rect.top + rect.height; row++) {
-        for (let column = rect.left; column < rect.left + rect.width; column++) {
-          if (routeCells.has(`${column}:${row}`)) return false
-        }
-      }
-      return true
+      return space.isFree(labelClaim(x, y), {
+        clearance: {
+          body: statePadding,
+          label: { x: 1, y: 0 },
+        },
+      })
     }
 
     let x = plan.label.x
@@ -672,7 +846,7 @@ function placeStateTransitionLabels(
       }
     }
 
-    placedLabels.push(labelRect({ ...plan.label, x, y }, width))
+    space = space.add(labelClaim(x, y))
     return { ...plan, label: { ...plan.label, x, y } }
   })
 }
@@ -703,6 +877,7 @@ export function createStateTransitionJunctionPlans(
   bounds: ReadonlyMap<string, BoxBounds>,
   renderPlans: readonly StateTransitionRenderPlan[],
 ): StateTransitionJunctionPlan[] {
+  const renderPlanByTransition = new Map(renderPlans.map((plan) => [plan.route.transition, plan]))
   return diagram.states.flatMap((state): StateTransitionJunctionPlan[] => {
     const kind =
       state.kind === "choice" ? "choice" : isHiddenCompositeMarker(state) ? "hidden-composite-marker" : undefined
@@ -713,7 +888,7 @@ export function createStateTransitionJunctionPlans(
     const connections = new Set<DiagramDirection>()
     const transitions: StateVisibleTransition[] = []
     for (const transition of diagram.transitions) {
-      const renderPlan = renderPlans.find((plan) => plan.route.transition === transition)
+      const renderPlan = renderPlanByTransition.get(transition)
       let connected = false
       if (transition.to === state.id) {
         const junction = renderPlan?.path.at(-1)

packages/merman/src/state/visible-model.test.ts

@@ -20,6 +20,43 @@ describe("prepareVisibleStateDiagram", () => {
     expect(visible.states.some((state) => state.id === "Authenticated.__start")).toBe(false)
     expect(visible.states.some((state) => state.id === "Authenticated.__end")).toBe(false)
     expect(entry).toMatchObject({ from: "__start", to: "Idle", label: "login" })
-    expect(exit).toMatchObject({ from: "Editing", to: "__end", label: "save" })
+    expect(exit).toMatchObject({ from: "Editing", to: "__end", label: "save<br/>logout" })
+  })
+
+  test("collapses nested composite entry chains without retaining scoped markers", () => {
+    const visible = prepareVisibleStateDiagram(
+      parseMermaidStateDiagram(`stateDiagram-v2
+  state Session {
+    [*] --> Open
+    state Open {
+      [*] --> Clean
+      Clean --> Dirty: edit
+      Dirty --> Clean: save
+    }
+    Open --> [*]: close
+  }
+  [*] --> Session
+  Session --> [*]`),
+    )
+
+    expect(visible.states.map((state) => state.id)).toEqual(["Clean", "Dirty", "__start", "__end"])
+    expect(visible.transitions).toContainEqual({ from: "__start", to: "Clean", label: "" })
+    expect(visible.transitions.some((transition) => transition.from.includes(".__start"))).toBe(false)
+    expect(visible.transitions.some((transition) => transition.to.includes(".__start"))).toBe(false)
+  })
+
+  test("preserves labels on both sides of collapsed composite markers", () => {
+    const visible = prepareVisibleStateDiagram(
+      parseMermaidStateDiagram(`stateDiagram-v2
+  [*] --> Session: open session
+  state Session {
+    [*] --> Ready: initialize
+    Ready --> [*]: finalize
+  }
+  Session --> [*]: close session`),
+    )
+
+    expect(visible.transitions).toContainEqual({ from: "__start", to: "Ready", label: "open session<br/>initialize" })
+    expect(visible.transitions).toContainEqual({ from: "Ready", to: "__end", label: "finalize<br/>close session" })
   })
 })

packages/merman/src/state/visible-model.ts

@@ -11,7 +11,7 @@ export function isHiddenCompositeMarker(state: StateDiagramState | undefined): b
 }
 
 function composeTransitionLabel(incoming: StateDiagramTransition, outgoing: StateDiagramTransition): string {
-  return incoming.label || outgoing.label
+  return [incoming.label, outgoing.label].filter(Boolean).join("<br/>")
 }
 
 function collapseHiddenCompositeMarkerTransitionsOnce(
@@ -23,33 +23,28 @@ function collapseHiddenCompositeMarkerTransitionsOnce(
   )
   if (hiddenMarkers.size === 0) return { transitions: [...transitions], changed: false }
 
-  const skipped = new Set<StateVisibleTransition>()
-  const collapsed: StateVisibleTransition[] = []
-  let changed = false
-
   for (const markerId of hiddenMarkers) {
     const incoming = transitions.filter((transition) => transition.to === markerId && transition.from !== markerId)
     const outgoing = transitions.filter((transition) => transition.from === markerId && transition.to !== markerId)
     if (incoming.length === 0 || outgoing.length === 0) continue
 
-    changed = true
-    for (const incomingTransition of incoming) {
-      skipped.add(incomingTransition)
-      for (const outgoingTransition of outgoing) {
-        skipped.add(outgoingTransition)
-        collapsed.push({
-          from: incomingTransition.from,
-          to: outgoingTransition.to,
-          label: composeTransitionLabel(incomingTransition, outgoingTransition),
-        })
-      }
+    const skipped = new Set([...incoming, ...outgoing])
+    return {
+      transitions: [
+        ...transitions.filter((transition) => !skipped.has(transition)),
+        ...incoming.flatMap((incomingTransition) =>
+          outgoing.map((outgoingTransition) => ({
+            from: incomingTransition.from,
+            to: outgoingTransition.to,
+            label: composeTransitionLabel(incomingTransition, outgoingTransition),
+          })),
+        ),
+      ],
+      changed: true,
     }
   }
 
-  return {
-    transitions: [...transitions.filter((transition) => !skipped.has(transition)), ...collapsed],
-    changed,
-  }
+  return { transitions: [...transitions], changed: false }
 }
 
 function collapseHiddenCompositeMarkerTransitions(diagram: StateDiagram): StateVisibleTransition[] {

packages/merman/src/test/diagnostics.test.ts

@@ -26,6 +26,21 @@ describe("parser diagnostics", () => {
     ).toThrow('Unsupported syntax in flowchart diagram at line 3: "A --o B"')
   })
 
+  test("does not partially parse unsupported flowchart syntax", () => {
+    for (const statement of ["A & B --> C", "A((Start)) --> B", "A-->B; B-->C"]) {
+      expect(() => parseMermaidFlowchartDiagram(`flowchart LR\n  ${statement}`)).toThrow(MermaidSyntaxError)
+    }
+  })
+
+  test("does not treat arrows inside flowchart node labels as edges", () => {
+    const diagram = parseMermaidFlowchartDiagram(`flowchart LR
+  A["send --> receive"] --> B`)
+
+    expect(diagram.nodes.map((node) => node.id)).toEqual(["A", "B"])
+    expect(diagram.nodes[0]?.label).toBe("send --> receive")
+    expect(diagram.edges).toHaveLength(1)
+  })
+
   test("exposes structured syntax errors through top-level rendering", () => {
     try {
       renderSequenceDiagram(`sequenceDiagram
@@ -41,6 +56,12 @@ describe("parser diagnostics", () => {
     }
   })
 
+  test("rejects unsupported bidirectional sequence arrows without phantom participants", () => {
+    for (const message of ["A<<->>B: hello", "A<<-->>B: hello"]) {
+      expect(() => parseMermaidSequenceDiagram(`sequenceDiagram\n  ${message}`)).toThrow(MermaidSyntaxError)
+    }
+  })
+
   test("reports unclosed state constructs at their opening line", () => {
     expect(() =>
       parseMermaidStateDiagram(`stateDiagram-v2
@@ -55,6 +76,17 @@ describe("parser diagnostics", () => {
     )
   })
 
+  test("rejects unsupported composite-local state directions", () => {
+    expect(() =>
+      parseMermaidStateDiagram(`stateDiagram-v2
+  direction LR
+  state Parent {
+    direction TB
+    A --> B
+  }`),
+    ).toThrow("Composite-local direction is not supported")
+  })
+
   test("reports malformed sequence block endings", () => {
     expect(() =>
       parseMermaidSequenceDiagram(`sequenceDiagram