fix(agent): 切回看过的会话直接铺出历史,长内容不再从顶滚到底

- 事件流按会话缓存最近 30 条会话的已读事件,切回时直接用缓存渲染,再从最后一个 seq 之后补读
- 消息区跟随:一次多出一屏以上直接跳到底,流式小段增长才平滑跟随
  (原来只有第一次 ResizeObserver 回调算首屏,事件补读回来时整段平滑滚动)

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Cxwgnn2pbzQ7rXv3rq7fei

apps/web/app/components/agent/use-agent-stream.ts

@@ -284,20 +284,49 @@ export function foldEvents(events: AgentRunEvent[]): StreamItem[] {
   return items;
 }
 
+/**
+ * 最近看过的会话的事件(模块级,最多 30 条会话):切回看过的会话时直接用已读到的事件铺出来,
+ * 再从最后一个 seq 之后补读;不再先空一帧、再整段补读回来。
+ */
+const EVENT_CACHE_MAX = 30;
+const eventCache = new Map<string, AgentRunEvent[]>();
+
+export function cachedRunEvents(key: string): AgentRunEvent[] {
+  return eventCache.get(key) ?? [];
+}
+
+export function cacheRunEvents(key: string, events: AgentRunEvent[]): void {
+  if (!key || !events.length) return;
+  eventCache.delete(key);
+  eventCache.set(key, events);
+  while (eventCache.size > EVENT_CACHE_MAX) {
+    const oldest = eventCache.keys().next().value;
+    if (oldest === undefined) break;
+    eventCache.delete(oldest);
+  }
+}
+
+const lastSeqOf = (events: AgentRunEvent[]) => events[events.length - 1]?.seq ?? -1;
+
 export function useAgentStream(runId: string | undefined, active: boolean): AgentStreamState {
-  const [events, setEvents] = useState<AgentRunEvent[]>([]);
+  // 演示数据与真实会话分开缓存
+  const cacheKey = runId ? `${agentMockEnabled() ? "mock" : "live"}:${runId}` : "";
+  const [events, setEvents] = useState<AgentRunEvent[]>(() => cachedRunEvents(cacheKey));
   const [error, setError] = useState<string | null>(null);
   const [connected, setConnected] = useState<AgentStreamState["connected"]>("idle");
   const [nonce, setNonce] = useState(0);
-  const lastSeq = useRef(-1);
+  const lastSeq = useRef(lastSeqOf(events));
 
-  // biome-ignore lint/correctness/useExhaustiveDependencies: 换 run 时重置
+  // biome-ignore lint/correctness/useExhaustiveDependencies: 换 run 时重置(有缓存就从缓存接着读)
   useEffect(() => {
-    setEvents([]);
+    const cached = cachedRunEvents(cacheKey);
+    setEvents(cached);
     setError(null);
-    lastSeq.current = -1;
+    lastSeq.current = lastSeqOf(cached);
   }, [runId]);
 
+  useEffect(() => cacheRunEvents(cacheKey, events), [cacheKey, events]);
+
   // biome-ignore lint/correctness/useExhaustiveDependencies: nonce 变化 = 手动重试,重新建立补读
   useEffect(() => {
     if (!runId) return;