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/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>
       )}