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);
+  });
+});

client/src/components/run/ChatOutline.tsx

@@ -16,7 +16,14 @@
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
-import { useEffect, useState, type ComponentType, type DragEvent, type ReactNode } from "react";
+import {
+  useEffect,
+  useState,
+  type ComponentType,
+  type DragEvent,
+  type ReactNode,
+  type RefObject,
+} from "react";
 import { useTranslation } from "react-i18next";
 import { Link } from "react-router-dom";
 import { useSettingsHref } from "../../lib/settingsLink";
@@ -35,6 +42,8 @@ import {
   X,
 } from "lucide-react";
 import { api, type SessionOutline } from "../../lib/api";
+import { keepIfSame } from "../../lib/keepIfSame";
+import { isShown } from "../../hooks/useOnScreen";
 import { displayName, extOf } from "./ChatFilePreview";
 import { dragAttachments, dragSkill, type Attachment } from "../../lib/promptAttach";
 
@@ -49,25 +58,30 @@ const EMPTY: SessionOutline = {
   mcp: [],
 };
 
-/** The session's outline, re-read on a visibility-gated poll. */
-export function useSessionOutline(sessionId: string): SessionOutline {
+/** The session's outline, re-read on a visibility-gated poll — skipped too
+ *  while `onScreen` says the pane is hidden (another workspace or tab, kept
+ *  mounted). An unchanged answer keeps the old object: no re-render. */
+export function useSessionOutline(
+  sessionId: string,
+  onScreen?: RefObject<boolean>
+): SessionOutline {
   const [outline, setOutline] = useState<SessionOutline>(EMPTY);
   useEffect(() => {
     let live = true;
     const load = () =>
       api.sessions
         .outline(sessionId)
-        .then((o) => live && setOutline({ ...EMPTY, ...o }))
+        .then((o) => live && setOutline(keepIfSame<SessionOutline>({ ...EMPTY, ...o })))
         .catch(() => {});
     void load();
     const timer = window.setInterval(() => {
-      if (document.visibilityState === "visible") void load();
+      if (onScreen ? isShown(onScreen) : document.visibilityState === "visible") void load();
     }, POLL_MS);
     return () => {
       live = false;
       window.clearInterval(timer);
     };
-  }, [sessionId]);
+  }, [sessionId, onScreen]);
   return outline;
 }
 
@@ -271,7 +285,7 @@ export function ChatOutline({
                   </span>
                 ) : (
                   <span
-                    className={`h-4 w-4 flex-shrink-0 rounded-full border-2 ${task.status === "in_progress" ? "animate-pulse border-accent" : "border-border"}`}
+                    className={`h-4 w-4 flex-shrink-0 rounded-full border-2 ${task.status === "in_progress" ? "animate-pulse-dot border-accent" : "border-border"}`}
                   />
                 )}
                 <span

client/src/components/run/LaneConsolePane.tsx

@@ -20,7 +20,7 @@
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
-import { useEffect, useRef, useState } from "react";
+import { useCallback, useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { AlertCircle, Plus } from "lucide-react";
 import { api } from "../../lib/api";
@@ -89,6 +89,13 @@ export function LaneConsolePane({
     : null;
   const chat = useSessionChat(chatSession);
   const chatable = !!handle && !!chatSession && CHAT_TOOLS.has(handle.tool);
+  // Stable, so the memoized terminal is not re-rendered for a new closure.
+  const onRunExitRef = useRef(onRunExit);
+  onRunExitRef.current = onRunExit;
+  const onTerminalExit = useCallback(() => {
+    setHandle(null);
+    onRunExitRef.current?.();
+  }, []);
 
   useEffect(() => {
     onHasActiveRunChange?.(handle !== null);
@@ -210,13 +217,11 @@ export function LaneConsolePane({
             runId={handle.id}
             wsBaseUrl={window.location.origin.replace(/^http/, "ws")}
             autoFocus={autoFocus && !(chatable && chat)}
+            covered={!!(chatable && chat)}
             // The pane died (double Ctrl+C out of `claude`, `exit`, a crash) —
             // the tab bar moves on (another tab, or the home's prompt box)
             // rather than leaving a dead terminal the user has to click out of.
-            onExit={() => {
-              setHandle(null);
-              onRunExit?.();
-            }}
+            onExit={onTerminalExit}
           />
         </div>
       )}

client/src/components/run/PaneChat.tsx

@@ -54,7 +54,7 @@
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
-import { useCallback, useEffect, useRef, useState, type CSSProperties } from "react";
+import { memo, useCallback, useEffect, useRef, useState, type CSSProperties } from "react";
 import { useTranslation } from "react-i18next";
 import { ArrowUp, FileText, Square, SquareCheck } from "lucide-react";
 import { isSubmitEnter } from "../../lib/keys";
@@ -76,6 +76,7 @@ import {
   type PaneMenu,
 } from "../../lib/panePaste";
 import { useLiveRefresh } from "../../hooks/useLiveRefresh";
+import { useOnScreen, isShown } from "../../hooks/useOnScreen";
 import { openInChanges } from "../../lib/openInChanges";
 import { toast } from "../../lib/toast";
 import { confirmDialog } from "../../lib/confirm";
@@ -306,7 +307,10 @@ function PaneMenuCard({
   );
 }
 
-export function PaneChat({
+/** Memoized: a lane_update re-renders the Workspace's pane tree, and every
+ *  prop here is a primitive or the pane's own run handle — nothing in a
+ *  frame about another lane reaches the chat. */
+export const PaneChat = memo(function PaneChat({
   run,
   sessionId,
   cwd,
@@ -370,7 +374,9 @@ export function PaneChat({
     return () => ro.disconnect();
   }, []);
   const [list, setList] = useState<OutlineList | null>(null);
-  const activity = useSessionOutline(sessionId);
+  // Kept mounted in a hidden workspace or tab: its pollers skip until shown.
+  const onScreen = useOnScreen(rootRef);
+  const activity = useSessionOutline(sessionId, onScreen);
   // Open by default only in a spanning pane, and only once there is an output.
   const outlineOpen = outlineChoice ?? (panelOpen && activity.outputs.length > 0);
   const scope: PreviewScope = { laneId, laneCwd: laneCwd || cwd, cwd, sessionId };
@@ -421,9 +427,12 @@ export function PaneChat({
         return JSON.stringify(next) === JSON.stringify(prev) ? prev : next;
       });
     read();
-    const timer = window.setInterval(read, 700);
+    // Nobody can answer a menu in a hidden tab or pane: no screen scan then.
+    const timer = window.setInterval(() => {
+      if (isShown(onScreen)) read();
+    }, 700);
     return () => window.clearInterval(timer);
-  }, [runId]);
+  }, [runId, onScreen]);
   const asking = waiting || menu !== null;
   // Gone until the pane redraws: no second press on a stale menu.
   const choose = (index: number) => {
@@ -451,7 +460,10 @@ export function PaneChat({
     model: null,
   });
   const onContext = useCallback(
-    (tokens: number, model: string | null) => setContext({ tokens, model }),
+    (tokens: number, model: string | null) =>
+      setContext((prev) =>
+        prev.tokens === tokens && prev.model === model ? prev : { tokens, model }
+      ),
     []
   );
 
@@ -678,4 +690,4 @@ export function PaneChat({
       )}
     </div>
   );
-}
+});

client/src/components/run/TerminalView.tsx

@@ -42,6 +42,7 @@
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 import {
+  memo,
   useEffect,
   useId,
   useMemo,
@@ -153,9 +154,22 @@ interface TerminalViewProps {
   /** Take the keyboard when it appears (default). Off for a split pane that
    *  is not the focused one, or keys would land in whichever mounted last. */
   autoFocus?: boolean;
+  /** Another layer (the pane's chat) covers it. xterm stops drawing only when
+   *  its IntersectionObserver says off screen, and a covered terminal is not —
+   *  so it is moved out of the viewport with a transform, which keeps its
+   *  layout size (no PTY resize; the chat still reads and answers its menus). */
+  covered?: boolean;
 }
 
-export function TerminalView({ runId, wsBaseUrl, onExit, autoFocus = true }: TerminalViewProps) {
+/** Memoized: the Workspace re-renders its pane tree on every lane_update, and
+ *  a terminal only cares about its own props (its owner keeps `onExit` stable). */
+export const TerminalView = memo(function TerminalView({
+  runId,
+  wsBaseUrl,
+  onExit,
+  autoFocus = true,
+  covered = false,
+}: TerminalViewProps) {
   const containerRef = useRef<HTMLDivElement>(null);
   // Read through a ref: an inline `onExit` from the parent changes identity on
   // every render, and as an effect dep it would tear the terminal down and
@@ -709,7 +723,12 @@ export function TerminalView({ runId, wsBaseUrl, onExit, autoFocus = true }: Ter
       onPasteCapture={onPaste}
       onFocus={() => notePaneFocus(runId)}
     >
-      <div ref={containerRef} className="h-full w-full" data-testid="terminal-view" />
+      <div
+        ref={containerRef}
+        className="h-full w-full"
+        style={covered ? { transform: "translateX(-300vw)" } : undefined}
+        data-testid="terminal-view"
+      />
       <p id={hintId} className="sr-only">
         {t("terminal.keyboardHint")}
       </p>
@@ -810,4 +829,4 @@ export function TerminalView({ runId, wsBaseUrl, onExit, autoFocus = true }: Ter
       )}
     </div>
   );
-}
+});

client/src/hooks/__tests__/useOnScreen.test.tsx

@@ -0,0 +1,56 @@
+/**
+ * @file useOnScreen.test.tsx
+ * @description useOnScreen follows the IntersectionObserver: on screen by
+ * default (and always without an observer, as in jsdom), off once the element
+ * stops intersecting, and `onShow` runs only on the way back on screen.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { act, render } from "@testing-library/react";
+import { useRef } from "react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { useOnScreen, isShown } from "../useOnScreen";
+
+const original = globalThis.IntersectionObserver;
+afterEach(() => {
+  globalThis.IntersectionObserver = original;
+});
+
+let seen: { current: boolean | null } = { current: null };
+function Probe({ onShow }: { onShow?: () => void }) {
+  const ref = useRef<HTMLDivElement>(null);
+  seen = useOnScreen(ref, onShow);
+  return <div ref={ref} />;
+}
+
+describe("useOnScreen", () => {
+  it("counts as on screen without an IntersectionObserver", () => {
+    // @ts-expect-error removed for the test
+    delete globalThis.IntersectionObserver;
+    render(<Probe />);
+    expect(seen.current).toBe(true);
+    expect(isShown(seen as never)).toBe(true);
+  });
+
+  it("follows the observer and calls onShow only when it comes back", () => {
+    let fire: (v: boolean) => void = () => {};
+    // @ts-expect-error test stub
+    globalThis.IntersectionObserver = class {
+      constructor(cb: (e: { isIntersecting: boolean }[]) => void) {
+        fire = (v) => cb([{ isIntersecting: v }]);
+      }
+      observe() {}
+      disconnect() {}
+    };
+    const onShow = vi.fn();
+    render(<Probe onShow={onShow} />);
+    act(() => fire(true));
+    expect(onShow).not.toHaveBeenCalled();
+    act(() => fire(false));
+    expect(seen.current).toBe(false);
+    expect(isShown(seen as never)).toBe(false);
+    act(() => fire(true));
+    expect(seen.current).toBe(true);
+    expect(onShow).toHaveBeenCalledTimes(1);
+  });
+});

client/src/hooks/useOnScreen.ts

@@ -0,0 +1,42 @@
+/**
+ * @file useOnScreen — whether an element is laid out on screen right now, for
+ * work that only matters while it can be seen. The Workspace keeps panes of
+ * other workspaces and other tabs MOUNTED but `display:none` (unmounting would
+ * drop a live terminal), so `document.visibilityState` alone keeps every hidden
+ * pane polling and re-rendering as if it were shown. An IntersectionObserver
+ * reports a `display:none` subtree as not intersecting; `onShow` runs when the
+ * element comes back, so a caller can catch up once instead of all the time.
+ * Without IntersectionObserver (jsdom, very old browsers) the element counts
+ * as always on screen — the behaviour before this hook existed.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { useEffect, useRef, type RefObject } from "react";
+
+export function useOnScreen(
+  ref: RefObject<Element | null>,
+  onShow?: () => void
+): RefObject<boolean> {
+  const visible = useRef(true);
+  const showRef = useRef(onShow);
+  showRef.current = onShow;
+
+  useEffect(() => {
+    const el = ref.current;
+    if (!el || typeof IntersectionObserver === "undefined") return;
+    const io = new IntersectionObserver((entries) => {
+      const now = entries[entries.length - 1]!.isIntersecting;
+      const was = visible.current;
+      visible.current = now;
+      if (now && !was) showRef.current?.();
+    });
+    io.observe(el);
+    return () => io.disconnect();
+  }, [ref]);
+
+  return visible;
+}
+
+/** True while the page is shown AND the element is on screen. */
+export const isShown = (onScreen: RefObject<boolean>) =>
+  document.visibilityState === "visible" && onScreen.current !== false;