fix: always return result array from useStableRows instead of stale fallback

The fallback path returned Array.from(prev.values()) which iterated the
old map's insertion order rather than the current rows order. It also
always created a new array, defeating the intended reference-reuse
optimization. Returning result unconditionally is both correct and
simpler.

Author: cursoragent

apps/web/src/components/chat/MessagesTimeline.tsx

@@ -230,10 +230,7 @@ export const MessagesTimeline = memo(function MessagesTimeline({
   // from TimelineRowCtx, which propagates through LegendList's memo.
   const renderItem = useCallback(
     ({ item }: { item: MessagesTimelineRow }) => (
-      <div
-        className="mx-auto w-full min-w-0 max-w-3xl overflow-x-hidden"
-        data-timeline-root="true"
-      >
+      <div className="mx-auto w-full min-w-0 max-w-3xl overflow-x-hidden" data-timeline-root="true">
         <TimelineRowContent row={item} />
       </div>
     ),
@@ -402,9 +399,7 @@ function TimelineRowContent({ row }: { row: TimelineRow }) {
                 <div className="my-3 flex items-center gap-3">
                   <span className="h-px flex-1 bg-border" />
                   <span className="rounded-full border border-border bg-background px-2.5 py-1 text-[10px] uppercase tracking-[0.14em] text-muted-foreground/80">
-                    {ctx.completionSummary
-                      ? `Response • ${ctx.completionSummary}`
-                      : "Response"}
+                    {ctx.completionSummary ? `Response • ${ctx.completionSummary}` : "Response"}
                   </span>
                   <span className="h-px flex-1 bg-border" />
                 </div>
@@ -794,7 +789,6 @@ function useStableRows(rows: MessagesTimelineRow[]): MessagesTimelineRow[] {
   return useMemo(() => {
     const prev = prevById.current;
     const next = new Map<string, MessagesTimelineRow>();
-    let anyChanged = false;
 
     const result = rows.map((row) => {
       const prevRow = prev.get(row.id);
@@ -803,13 +797,11 @@ function useStableRows(rows: MessagesTimelineRow[]): MessagesTimelineRow[] {
         return prevRow;
       }
       next.set(row.id, row);
-      anyChanged = true;
       return row;
     });
 
     prevById.current = next;
-    // If nothing changed and length matches, reuse the previous array reference
-    return anyChanged || rows.length !== prev.size ? result : Array.from(prev.values());
+    return result;
   }, [rows]);
 }