fix(chat): prevent UI flicker when streaming response finalizes

The live timeline was rendered in a separate <div> from the persisted
messages list. When the streamed task finished and its ChatMessage
landed, the live <div> unmounted and a new <MessageBubble> mounted —
two different DOM elements showing the same content. useAutoScroll's
ResizeObserver + MutationObserver fired on both the unmount and the
mount, causing the visible jump-then-re-render.

Merge the two paths: inject a synthetic assistant message with the
pending task_id while streaming, and key every assistant bubble by
task_id. When the real message arrives (same task_id), React preserves
the DOM element across the invalidate → refetch window — no remount,
no double scroll, no flicker.

packages/views/chat/components/chat-message-list.tsx

@@ -35,23 +35,26 @@ export function ChatMessageList({
   const fadeStyle = useScrollFade(scrollRef);
   useAutoScroll(scrollRef);
 
-  // Once the assistant message for this pending task has landed in the
-  // messages list, AssistantMessage owns its rendering — suppress the live
-  // timeline to avoid rendering the same content in two places during the
-  // invalidate → refetch window.
+  // While a task is in flight and its assistant message hasn't landed yet,
+  // inject a synthetic placeholder carrying the same task_id. AssistantMessage
+  // reads the live timeline from the shared taskMessagesOptions cache (fed by
+  // WS), so the synthetic bubble renders exactly what the persisted bubble
+  // will. Because MessageBubble is keyed by task_id when present, the DOM
+  // element is preserved when the real message arrives — no unmount/remount,
+  // no double scroll-to-bottom, no visible jump.
   const pendingAlreadyPersisted = !!pendingTaskId && messages.some(
     (m) => m.role === "assistant" && m.task_id === pendingTaskId,
   );
-
-  // Live timeline for the in-flight task. useRealtimeSync keeps this cache
-  // current via setQueryData on task:message events.
-  const showLiveTimeline = !!pendingTaskId && !pendingAlreadyPersisted;
-  const { data: liveTaskMessages } = useQuery({
-    ...taskMessagesOptions(pendingTaskId ?? ""),
-    enabled: showLiveTimeline,
-  });
-  const liveTimeline: ChatTimelineItem[] = (liveTaskMessages ?? []).map(toTimelineItem);
-  const hasLive = showLiveTimeline && liveTimeline.length > 0;
+  const displayMessages: ChatMessage[] = pendingTaskId && !pendingAlreadyPersisted
+    ? [...messages, {
+        id: `pending-${pendingTaskId}`,
+        chat_session_id: messages[messages.length - 1]?.chat_session_id ?? "",
+        role: "assistant",
+        content: "",
+        task_id: pendingTaskId,
+        created_at: new Date().toISOString(),
+      }]
+    : messages;
 
   return (
     <div ref={scrollRef} style={fadeStyle} className="flex-1 overflow-y-auto">
@@ -60,15 +63,14 @@ export function ChatMessageList({
        *  views doesn't jolt the reading width. px-5 is a touch tighter
        *  than issue-detail's px-8 because the chat window can be narrow. */}
       <div className="mx-auto w-full max-w-4xl px-5 py-4 space-y-4">
-        {messages.map((msg) => (
-          <MessageBubble key={msg.id} message={msg} />
+        {displayMessages.map((msg) => (
+          <MessageBubble
+            key={msg.role === "assistant" && msg.task_id ? `task-${msg.task_id}` : msg.id}
+            message={msg}
+            isPending={!!pendingTaskId && msg.task_id === pendingTaskId && !pendingAlreadyPersisted}
+          />
         ))}
-        {hasLive && (
-          <div className="w-full space-y-1.5">
-            <TimelineView items={liveTimeline} />
-          </div>
-        )}
-        {isWaiting && !hasLive && !pendingAlreadyPersisted && (
+        {isWaiting && !pendingTaskId && (
           <Loader2 className="size-4 animate-spin text-muted-foreground" />
         )}
       </div>
@@ -116,7 +118,7 @@ function toTimelineItem(m: TaskMessagePayload): ChatTimelineItem {
 
 // ─── Message bubbles ─────────────────────────────────────────────────────
 
-function MessageBubble({ message }: { message: ChatMessage }) {
+function MessageBubble({ message, isPending }: { message: ChatMessage; isPending?: boolean }) {
   if (message.role === "user") {
     return (
       <div className="flex justify-end">
@@ -133,13 +135,15 @@ function MessageBubble({ message }: { message: ChatMessage }) {
     );
   }
 
-  return <AssistantMessage message={message} />;
+  return <AssistantMessage message={message} isPending={isPending} />;
 }
 
 function AssistantMessage({
   message,
+  isPending,
 }: {
   message: ChatMessage;
+  isPending?: boolean;
 }) {
   const taskId = message.task_id;
 
@@ -157,11 +161,13 @@ function AssistantMessage({
     <div className="w-full space-y-1.5">
       {timeline.length > 0 ? (
         <TimelineView items={timeline} />
-      ) : (
+      ) : message.content ? (
         <div className="text-sm leading-relaxed prose prose-sm dark:prose-invert max-w-none">
           <Markdown>{message.content}</Markdown>
         </div>
-      )}
+      ) : isPending ? (
+        <Loader2 className="size-4 animate-spin text-muted-foreground" />
+      ) : null}
     </div>
   );
 }