fix(webui): preserve semantic history anchors

webui/src/components/thread/ThreadMessages.tsx

@@ -136,6 +136,7 @@ export function ThreadMessages({
         return (
           <ThreadDisplayUnit
             key={unitKeys[index]}
+            unitKey={unitKeys[index]}
             unit={unit}
             marginTop={marginTop}
             userPromptId={userPromptId}
@@ -225,6 +226,7 @@ function pendingTurnProjection(
 }
 
 interface ThreadDisplayUnitProps {
+  unitKey: string;
   unit: DisplayUnit;
   marginTop: string;
   userPromptId?: string;
@@ -243,6 +245,7 @@ interface ThreadDisplayUnitProps {
 }
 
 const ThreadDisplayUnit = memo(function ThreadDisplayUnit({
+  unitKey,
   unit,
   marginTop,
   userPromptId,
@@ -273,7 +276,7 @@ const ThreadDisplayUnit = memo(function ThreadDisplayUnit({
     <>
       <div
         className={`${marginTop}${stableDeferOffscreenRender ? " thread-render-unit" : ""}`}
-        data-thread-display-unit
+        data-thread-display-unit={unitKey}
         data-user-prompt-id={userPromptId}
       >
         {unit.type === "activity" ? (

webui/src/components/thread/ThreadViewport.tsx

@@ -73,7 +73,7 @@ export const INITIAL_HISTORY_WINDOW = 160;
 export const HISTORY_WINDOW_INCREMENT = 120;
 
 interface HistoryScrollAnchor {
-  element: HTMLElement;
+  key: string;
   offsetTop: number;
 }
 
@@ -316,8 +316,10 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
     for (const element of units) {
       const bounds = element.getBoundingClientRect();
       if (bounds.bottom <= scrollerTop + 0.5) continue;
+      const key = element.dataset.threadDisplayUnit;
+      if (!key) continue;
       historyScrollAnchorRef.current = {
-        element,
+        key,
         offsetTop: bounds.top - scrollerTop,
       };
       return true;
@@ -328,15 +330,19 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
 
   const reconcileHistoryScrollAnchor = useCallback(() => {
     const scroller = scrollRef.current;
+    const content = messageContentRef.current;
     const anchor = historyScrollAnchorRef.current;
-    if (!scroller || !anchor) return false;
-    if (!anchor.element.isConnected) {
+    if (!scroller || !content || !anchor) return false;
+    const element = Array.from(
+      content.querySelectorAll<HTMLElement>("[data-thread-display-unit]"),
+    ).find((candidate) => candidate.dataset.threadDisplayUnit === anchor.key);
+    if (!element) {
       historyScrollAnchorRef.current = null;
       return false;
     }
 
     const nextOffset =
-      anchor.element.getBoundingClientRect().top
+      element.getBoundingClientRect().top
       - scroller.getBoundingClientRect().top;
     const delta = nextOffset - anchor.offsetTop;
     if (Math.abs(delta) < 0.5) return true;
@@ -344,7 +350,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
     const nextTop = Math.min(maxScrollTop, Math.max(0, scroller.scrollTop + delta));
     threadMotionRef.current?.jumpTo(nextTop);
     return true;
-  }, [captureHistoryScrollAnchor]);
+  }, []);
 
   const scrollToBottomNow = useCallback((smooth = false) => {
     historyScrollAnchorRef.current = null;

webui/src/tests/thread-viewport.test.tsx

@@ -1521,8 +1521,18 @@ describe("ThreadViewport", () => {
         dispatchUserScroll(scroller);
       });
 
+      const replacement = anchor.cloneNode(true) as HTMLElement;
+      anchor.replaceWith(replacement);
       anchorDocumentTop += 180;
       scrollHeight += 180;
+      Object.defineProperty(replacement, "getBoundingClientRect", {
+        configurable: true,
+        value: () => DOMRect.fromRect({
+          y: anchorDocumentTop - scroller.scrollTop,
+          width: 800,
+          height: 40,
+        }),
+      });
       const content = screen.getByTestId("thread-message-region").firstElementChild;
       const observer = resizeObserver.observers.find((candidate) =>
         content ? candidate.elements.includes(content) : false,
@@ -1533,7 +1543,7 @@ describe("ThreadViewport", () => {
       });
 
       expect(scroller.scrollTop).toBe(260);
-      expect(anchor.getBoundingClientRect().top).toBe(120);
+      expect(replacement.getBoundingClientRect().top).toBe(120);
     } finally {
       resizeObserver.restore();
     }