ui: fix sticky session diffs header (#21486)

packages/ui/src/components/session-turn.css

@@ -94,9 +94,15 @@
 
   [data-slot="session-turn-diffs-header"] {
     display: flex;
-    align-items: baseline;
+    align-items: center;
     gap: 8px;
+    padding-top: 4px;
     padding-bottom: 12px;
+    position: sticky;
+    top: var(--sticky-accordion-top, 0px);
+    z-index: 20;
+    background-color: var(--background-stronger);
+    height: 44px;
   }
 
   [data-slot="session-turn-diffs-label"] {

packages/ui/src/components/session-turn.tsx

@@ -447,7 +447,7 @@ export function SessionTurn(
                   <div data-component="session-turn-diffs-content">
                     <Accordion
                       multiple
-                      style={{ "--sticky-accordion-offset": "40px" }}
+                      style={{ "--sticky-accordion-offset": "44px" }}
                       value={expanded()}
                       onChange={(value) => setState("expanded", Array.isArray(value) ? value : value ? [value] : [])}
                     >