revert: restore stable chat history scrolling

packages/ui/src/components/chat/MessageList.tsx

@@ -1372,8 +1372,6 @@ const MessageList = React.forwardRef<MessageListHandle, MessageListProps>(({
         scrollEl.scrollTop += prependedHeight;
     });
 
-    const [historyVirtualRows, setHistoryVirtualRows] = React.useState<VirtualItem[]>(EMPTY_VIRTUAL_ROWS);
-
     const historyVirtualizer = useVirtualizer({
         count: historyEntries.length,
         getScrollElement: resolveScrollContainer,
@@ -1383,9 +1381,6 @@ const MessageList = React.forwardRef<MessageListHandle, MessageListProps>(({
         useAnimationFrameWithResizeObserver: true,
         overscan: MESSAGE_LIST_OVERSCAN,
         enabled: shouldVirtualizeHistory,
-        onChange: () => {
-            setHistoryVirtualRows(historyVirtualizer.getVirtualItems());
-        },
     });
 
     React.useLayoutEffect(() => {
@@ -1440,13 +1435,10 @@ const MessageList = React.forwardRef<MessageListHandle, MessageListProps>(({
         };
     }, []);
 
-    // Sync virtual rows on initial mount and when virtualization toggles.
+    const historyVirtualRows = React.useMemo(
-    // Ongoing updates are handled by the virtualizer's onChange callback.
+        () => (shouldVirtualizeHistory ? historyVirtualizer.getVirtualItems() : EMPTY_VIRTUAL_ROWS),
-    React.useLayoutEffect(() => {
+        [historyVirtualizer, shouldVirtualizeHistory],
-        setHistoryVirtualRows(
+    );
-            shouldVirtualizeHistory ? historyVirtualizer.getVirtualItems() : EMPTY_VIRTUAL_ROWS,
-        );
-    }, [shouldVirtualizeHistory, historyVirtualizer]);
 
     const allEntries = React.useMemo(() => {
         return trailingStreamingEntry ? [...historyEntries, trailingStreamingEntry] : historyEntries;

packages/ui/src/hooks/useChatAutoFollow.ts

@@ -411,9 +411,7 @@ export const useChatAutoFollow = ({
     }, [sessionIsWorking, startFollowLoop]);
 
     // Replay a deferred restoreSnapshot once ChatViewport mounts.
-    // useLayoutEffect ensures scroll position is set before the browser paints,
+    React.useEffect(() => {
-    // preventing a visible flash of content at the wrong scroll position.
-    React.useLayoutEffect(() => {
         if (!containerEl) return;
         if (pendingInitialRestoreRef.current && pendingInitialRestoreRef.current === currentSessionId) {
             void restoreSnapshot();