fix: prevent blank chat viewport when switching sessions (#1553)

When switching to a session with long context (especially in Electron
desktop when changing servers), the chat viewport could render blank
until the user scrolled. Two interacting issues caused this:

1. historyVirtualRows memo never recomputed after the first render
   because historyVirtualizer (from useVirtualizer's useState) is a
   stable reference. Frozen range meant items rendered at the top
   while paddingBottom filled the visible viewport after scrolling.

2. pendingInitialRestoreRef replay ran in useEffect (after paint),
   showing a frame at scrollTop:0 with the stale virtualizer range.

Fixed by: adding a virtualVersion counter driven by useVirtualizer's
onChange to bust the memo; switching the replay to useLayoutEffect
so scroll position is set before the browser paints.

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

@@ -1372,6 +1372,8 @@ const MessageList = React.forwardRef<MessageListHandle, MessageListProps>(({
         scrollEl.scrollTop += prependedHeight;
     });
 
+    const [virtualVersion, bumpVirtualVersion] = React.useReducer((v: number) => v + 1, 0);
+
     const historyVirtualizer = useVirtualizer({
         count: historyEntries.length,
         getScrollElement: resolveScrollContainer,
@@ -1381,6 +1383,7 @@ const MessageList = React.forwardRef<MessageListHandle, MessageListProps>(({
         useAnimationFrameWithResizeObserver: true,
         overscan: MESSAGE_LIST_OVERSCAN,
         enabled: shouldVirtualizeHistory,
+        onChange: bumpVirtualVersion,
     });
 
     React.useLayoutEffect(() => {
@@ -1437,7 +1440,7 @@ const MessageList = React.forwardRef<MessageListHandle, MessageListProps>(({
 
     const historyVirtualRows = React.useMemo(
         () => (shouldVirtualizeHistory ? historyVirtualizer.getVirtualItems() : EMPTY_VIRTUAL_ROWS),
-        [historyVirtualizer, shouldVirtualizeHistory],
+        [historyVirtualizer, shouldVirtualizeHistory, virtualVersion],
     );
 
     const allEntries = React.useMemo(() => {

packages/ui/src/hooks/useChatAutoFollow.ts

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