perf(chat): hidden panes stop polling, rows stop re-rendering, a covered terminal stops drawing

The Workspace keeps panes of other workspaces and tabs mounted (display:none),
and every one of them kept following the live stream as if shown.

- useOnScreen (IntersectionObserver): ConversationView, the outline and the
  700 ms menu scan skip while their pane is hidden and catch up once on show.
- A new_event refetches the tail at most every 1.5 s (leading + trailing).
- MessageList: WorkGroup/MessageRow/EditedFiles compare only their own blocks
  and results, so a tail append no longer re-renders every row.
- A terminal under the chat is moved off the viewport with a transform, so
  xterm pauses drawing without a resize.
- PaneChat and TerminalView are memoized; LaneConsolePane passes a stable onExit.
- RunningStep's clock and the outline poll skip unchanged or hidden updates.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/src/components/conversation/ConversationView.tsx

@@ -61,13 +61,15 @@
  *
  * ----------------------------------------------------------------------------- */
 
-import { useEffect, useState, useCallback, useRef } from "react";
+import { useEffect, useState, useCallback, useMemo, useRef } from "react";
 import { Loader2, ArrowDown, MessagesSquare, RefreshCw } from "lucide-react";
 import { api } from "../../lib/api";
 import { eventBus } from "../../lib/eventBus";
 import { isRemoteDataRefreshMessage } from "../../lib/remoteDataEvents";
 import { MessageList, promptTokens } from "./MessageList";
 import { useSpin } from "../../hooks/useSpin";
+import { useOnScreen, isShown } from "../../hooks/useOnScreen";
+import { throttle } from "../../lib/throttle";
 import { useTranslation } from "react-i18next";
 import type { TranscriptMessage, TranscriptInfo, WSMessage } from "../../lib/types";
 
@@ -80,6 +82,10 @@ const POLL_INTERVAL_MS = 3000;
 // Rescan the transcripts list periodically so new subagents that spawn
 // mid-session appear in the dropdown without a page reload.
 const TRANSCRIPTS_REFRESH_MS = 15000;
+// A hook burst (Pre + Post per tool call, several sessions) refetches the tail
+// at most this often: the first event still fetches at once, the last is never
+// lost, and a long transcript is not re-read for every single frame.
+const EVENT_FETCH_MS = 1500;
 
 interface ConversationViewProps {
   sessionId: string;
@@ -161,6 +167,10 @@ export function ConversationView({
   // Bumped whenever the session or the selected transcript changes: a tail
   // fetch started for the previous transcript must not append to the new one.
   const genRef = useRef(0);
+  // A pane of another workspace or tab stays mounted but hidden: it neither
+  // polls nor follows events, and catches up once when it is shown again.
+  const onShowRef = useRef<() => void>(() => {});
+  const onScreen = useOnScreen(scrollContainerRef, () => onShowRef.current());
   // Refresh-button spinner state - separate from initial `loading` so the
   // existing skeleton doesn't blink during a manual refresh.
 
@@ -179,7 +189,7 @@ export function ConversationView({
     }
     loadTranscripts();
     const interval = window.setInterval(() => {
-      if (document.visibilityState === "visible") loadTranscripts();
+      if (isShown(onScreen)) loadTranscripts();
     }, TRANSCRIPTS_REFRESH_MS);
     return () => {
       cancelled = true;
@@ -299,24 +309,33 @@ export function ConversationView({
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [sessionId, selectedTranscript]);
 
-  // WebSocket subscription: refetch on every new_event for this session.
+  // WebSocket subscription: refetch on new_event for this session, throttled.
   // Hook coverage isn't complete (a user-typed message fires no hook), so we
   // also poll below to catch what WS misses.
+  const fetchSoon = useMemo(
+    () => throttle(() => void fetchNewMessages(), EVENT_FETCH_MS),
+    [fetchNewMessages]
+  );
+  useEffect(() => () => fetchSoon.cancel(), [fetchSoon]);
   useEffect(() => {
     const unsubscribe = eventBus.subscribe((msg: WSMessage) => {
-      // Hidden tab: the visibility handler below catches up once on show.
-      if (document.visibilityState !== "visible") return;
+      // Hidden tab or hidden pane: catching up once on show is enough.
+      if (!isShown(onScreen)) return;
       if (isRemoteDataRefreshMessage(msg)) {
-        fetchNewMessages();
+        fetchSoon();
         return;
       }
       if (msg.type !== "new_event") return;
       const data = msg.data as { session_id?: string };
       if (data.session_id !== sessionId) return;
-      fetchNewMessages();
+      fetchSoon();
     });
     return unsubscribe;
-  }, [sessionId, fetchNewMessages]);
+  }, [sessionId, fetchSoon, onScreen]);
+
+  onShowRef.current = () => {
+    if (document.visibilityState === "visible") void fetchNewMessages();
+  };
 
   // Resync on WebSocket reconnect: events that landed during a transient
   // disconnect are gone from the bus, but the JSONL still has them, so a
@@ -337,7 +356,7 @@ export function ConversationView({
     function start() {
       if (interval !== null) return;
       interval = window.setInterval(() => {
-        if (document.visibilityState === "visible") fetchNewMessages();
+        if (isShown(onScreen)) fetchNewMessages();
       }, POLL_INTERVAL_MS);
     }
     function stop() {
@@ -363,7 +382,7 @@ export function ConversationView({
       stop();
       document.removeEventListener("visibilitychange", onVisibility);
     };
-  }, [fetchNewMessages]);
+  }, [fetchNewMessages, onScreen]);
 
   // Manual refresh - surfaces a control in the toolbar so users can force
   // a sync without reloading the page.

client/src/components/conversation/MessageList.tsx

@@ -179,6 +179,17 @@ function buildToolResultMap(messages: TranscriptMessage[]): Map<string, Transcri
   return map;
 }
 
+type ResultMap = ReturnType<typeof buildToolResultMap>;
+
+/** The map is rebuilt on every tail append, but a block's own result keeps its
+ *  identity (old messages are kept, not copied): comparing just those lets a
+ *  memoized row skip the re-render when nothing it shows changed. */
+const sameResults = (blocks: TranscriptContent[], a: ResultMap, b: ResultMap) =>
+  a === b || blocks.every((bl) => !bl.id || a.get(bl.id) === b.get(bl.id));
+
+const sameBlocks = (a: TranscriptContent[], b: TranscriptContent[]) =>
+  a === b || (a.length === b.length && a.every((bl, i) => bl === b[i]));
+
 /** The whole prompt a message was answered from: `input_tokens` is only its
  *  uncached tail (a few tokens once the cache is warm), the rest is cache. */
 export function promptTokens(u: NonNullable<TranscriptMessage["usage"]>): number {
@@ -544,7 +555,7 @@ export function chatItems(messages: TranscriptMessage[], keys: string[]): Item[]
 
 /** Claude Desktop's end-of-turn card: the files the turn changed, each with
  *  the lines added and removed. A call still running or failed is left out. */
-function EditedFiles({
+function EditedFilesCard({
   blocks,
   toolResultMap,
   onOpenFile,
@@ -614,6 +625,13 @@ function EditedFiles({
     </div>
   );
 }
+const EditedFiles = memo(
+  EditedFilesCard,
+  (p, n) =>
+    p.onOpenFile === n.onOpenFile &&
+    sameBlocks(p.blocks, n.blocks) &&
+    sameResults(n.blocks, p.toolResultMap, n.toolResultMap)
+);
 
 export function MessageList({
   messages,
@@ -870,8 +888,16 @@ function RunningStep({
   const { t } = useTranslation("sessions");
   const [now, setNow] = useState(() => Date.now());
   const [showAll, setShowAll] = useState(false);
+  const boxRef = useRef<HTMLDivElement>(null);
   useEffect(() => {
-    const timer = window.setInterval(() => setNow(Date.now()), 1000);
+    // A hidden tab or a hidden pane (another workspace, kept mounted) shows
+    // no clock: skip the re-render until it is on screen again.
+    const timer = window.setInterval(() => {
+      const el = boxRef.current;
+      if (document.visibilityState !== "visible" || (el && el.getClientRects().length === 0))
+        return;
+      setNow(Date.now());
+    }, 1000);
     return () => window.clearInterval(timer);
   }, []);
   const counted = useCountUp(tokens);
@@ -893,7 +919,7 @@ function RunningStep({
   ].filter(Boolean);
   const amber = waiting || quiet;
   return (
-    <div data-testid="chat-running-step" className="space-y-1 text-sm text-fg-muted">
+    <div ref={boxRef} data-testid="chat-running-step" className="space-y-1 text-sm text-fg-muted">
       <div className="flex flex-wrap items-center gap-x-2 gap-y-1">
         {waiting ? (
           <Hand className="h-3.5 w-3.5 flex-shrink-0 text-status-warning" />
@@ -1369,7 +1395,7 @@ function WorkStep({
 
 /** What the assistant did between two replies as one line, opening onto
  *  one framed list of its steps. */
-const WorkGroup = memo(function WorkGroup({
+function WorkGroupLine({
   blocks,
   toolResultMap,
 }: {
@@ -1391,11 +1417,16 @@ const WorkGroup = memo(function WorkGroup({
       </Disclosure>
     </div>
   );
-});
+}
+const WorkGroup = memo(
+  WorkGroupLine,
+  (p, n) =>
+    sameBlocks(p.blocks, n.blocks) && sameResults(n.blocks, p.toolResultMap, n.toolResultMap)
+);
 
 /** One user/assistant row. Memoized: a live tail append re-renders only the
  *  new rows; its thinking-block expansion is its own state, kept by key. */
-const MessageRow = memo(function MessageRow({
+function MessageRowView({
   msg,
   toolResultMap,
 }: {
@@ -1514,4 +1545,8 @@ const MessageRow = memo(function MessageRow({
       </div>
     </div>
   );
-});
+}
+const MessageRow = memo(
+  MessageRowView,
+  (p, n) => p.msg === n.msg && sameResults(n.msg.content, p.toolResultMap, n.toolResultMap)
+);

client/src/components/conversation/__tests__/ConversationView.hidden.test.tsx

@@ -0,0 +1,96 @@
+/**
+ * @file ConversationView.hidden.test.tsx
+ * @description A chat kept mounted in a hidden pane (another workspace or
+ * tab, `display:none`) does not follow the live stream: a new_event for its
+ * session fetches nothing while its box is off screen, and showing it again
+ * catches the tail up once. On screen, a burst of events is throttled to one
+ * fetch at once plus one after the burst.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { act, render, screen } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { ConversationView } from "../ConversationView";
+import { api } from "../../../lib/api";
+import { eventBus } from "../../../lib/eventBus";
+
+vi.mock("../../../lib/api", async (importOriginal) => {
+  const actual = await importOriginal<Record<string, unknown>>();
+  return {
+    ...actual,
+    api: {
+      sessions: {
+        transcripts: vi.fn(async () => ({ transcripts: [] })),
+        transcript: vi.fn(async () => ({
+          messages: [
+            {
+              type: "user",
+              content: [{ type: "text", text: "hello" }],
+              timestamp: "2026-10-09T00:00:00Z",
+            },
+          ],
+          total: 1,
+          has_more: false,
+          first_line: 1,
+          last_line: 1,
+        })),
+      },
+    },
+  };
+});
+
+let intersect: ((visible: boolean) => void) | null = null;
+const original = globalThis.IntersectionObserver;
+beforeEach(() => {
+  intersect = null;
+  // @ts-expect-error test stub
+  globalThis.IntersectionObserver = class {
+    constructor(cb: (e: { isIntersecting: boolean }[]) => void) {
+      intersect = (visible) => cb([{ isIntersecting: visible }]);
+    }
+    observe() {}
+    disconnect() {}
+  };
+});
+afterEach(() => {
+  globalThis.IntersectionObserver = original;
+  vi.useRealTimers();
+  vi.clearAllMocks();
+});
+
+const event = () =>
+  act(() => eventBus.publish({ type: "new_event", data: { session_id: "s1" } } as never));
+const calls = () => vi.mocked(api.sessions.transcript).mock.calls.length;
+
+describe("ConversationView in a hidden pane", () => {
+  it("fetches nothing while hidden and catches up once when shown", async () => {
+    render(<ConversationView sessionId="s1" fill live />);
+    await screen.findByText("hello");
+    const before = calls();
+
+    act(() => intersect?.(false));
+    event();
+    event();
+    expect(calls()).toBe(before);
+
+    act(() => intersect?.(true));
+    await act(async () => {});
+    expect(calls()).toBe(before + 1);
+  });
+
+  it("throttles a burst of events on screen", async () => {
+    render(<ConversationView sessionId="s1" fill live />);
+    await screen.findByText("hello");
+    vi.useFakeTimers();
+    const before = calls();
+    event();
+    event();
+    event();
+    // The first fetches at once; the rest wait for the throttle window.
+    expect(calls()).toBe(before + 1);
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(1600);
+    });
+    expect(calls()).toBe(before + 2);
+  });
+});