fix: message list shaking/flickering when scrolling to top or bottom

topic.onData = notify (wireTopicCallbacks) fires once per individual
message, not once per batch. Live-instrumented a real loadOlder() page
fetch (24 older messages): scrollHeight/scrollTop mutated in ~14
separate small steps over ~400ms, each one racing loadOlder()'s own
scroll-compensation and ConversationView's separate autoscroll-to-
bottom effect before the page had actually finished arriving -- that
rapid multi-step crawl is what reads as "shaking" rather than one
clean jump.

Coalesce notifications landing within the same animation frame into a
single flush, so a burst of onData calls (initial load, loadOlder(), a
fast run of incoming messages) triggers at most one re-render per
frame instead of N. messageSnapshots invalidation stays synchronous
(pure cache bookkeeping); only the listener callbacks that schedule
React's re-render are batched.

Live-verified with the same instrumentation: the same 24-message
loadOlder() fetch now produces 2 dom-mutation events instead of 14,
with scrollTop only moving once the batch has actually settled instead
of crawling in lockstep with a growing scrollHeight.

web/src/lib/tinode/topics.ts

@@ -86,10 +86,50 @@ export function markConfirmedRead(topicName: string, seq: number) {
   if (seq > prev) confirmedReadSeq.set(topicName, seq);
 }
 
+// Live-confirmed root cause of a real reported bug ("聊天消息框滚动到顶部或底部时UI会抖动闪烁"):
+// `topic.onData = notify` (wireTopicCallbacks below) fires once per INDIVIDUAL message, not once per
+// batch -- confirmed by instrumenting a real `loadOlder()` page fetch (24 older messages): the DOM
+// mutated in ~14 separate small steps over ~400ms, each one growing `scrollHeight` a little and each
+// one racing the OTHER open effect that reads it (loadOlder's own scroll-compensation, and
+// ConversationView's separate "autoscroll to bottom" effect) -- neither gets a stable, final
+// `scrollHeight` to work with until the whole page has actually finished arriving, so each of the ~14
+// intermediate re-renders nudges `scrollTop` again, which is what actually reads as "shaking" rather
+// than one clean jump. This wasn't reproducible by reasoning about a single `getMessagesPage()` call
+// in isolation -- it only showed up instrumenting `scrollTop` across a real fetch.
+// Fix: coalesce any notifications that land within the same animation frame into one flush, so a
+// burst of N `onData` calls (initial page load, `loadOlder()`, or a fast run of incoming messages)
+// triggers at most one re-render per frame instead of N -- imperceptible as a delay, but it means
+// `scrollHeight` is only ever read by a re-render once it reflects everything that arrived so far in
+// that frame, not some in-between partial state. `messageSnapshots.delete` stays synchronous (it's
+// pure cache invalidation, not a re-render) so a snapshot read between two notifications in the same
+// frame still sees the latest data, only the LISTENER callbacks (which is what schedules React's own
+// re-render) are what get batched.
+const pendingTopicNotifications = new Set<string>();
+let pendingAnyNotification = false;
+let notifyFrameScheduled = false;
+
+function flushTopicNotifications() {
+  notifyFrameScheduled = false;
+  const topics = [...pendingTopicNotifications];
+  pendingTopicNotifications.clear();
+  const flushAny = pendingAnyNotification;
+  pendingAnyNotification = false;
+  for (const topicName of topics) {
+    topicListeners.get(topicName)?.forEach((cb) => cb());
+  }
+  if (flushAny) {
+    anyTopicListeners.forEach((cb) => cb());
+  }
+}
+
 function notifyTopic(topicName: string) {
   messageSnapshots.delete(topicName);
-  topicListeners.get(topicName)?.forEach((cb) => cb());
-  anyTopicListeners.forEach((cb) => cb());
+  pendingTopicNotifications.add(topicName);
+  pendingAnyNotification = true;
+  if (!notifyFrameScheduled) {
+    notifyFrameScheduled = true;
+    requestAnimationFrame(flushTopicNotifications);
+  }
 }
 
 export function subscribeTopicChanges(topicName: string, callback: () => void): () => void {