perf(workspace): switching workspaces keeps their terminals mounted

Panes were keyed by slot alone, so slot 0 of the next workspace reused the
previous one's pane: every switch disposed its terminals and attached new
ones (a WebSocket each, the whole screen replayed). The live panes of the
last 4 workspaces left now stay mounted as hidden siblings in the same
grid, keyed by paneKey (lane x 10 + slot), so going back is instant.

Measured on 3 workspaces x 2-3 shell panes, switching back to one already
opened: script 210 -> 52 ms, main thread 370 -> 120 ms, long tasks
120 -> 0 ms, terminal re-attaches 2-3 -> 0.

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

client/src/components/run/PaneGrid.tsx

@@ -19,6 +19,9 @@
  * the close as one menu. Dividers resize by pointer or by arrow keys; a drag
  * moves only the line and commits the ratio on release, so the layout is
  * saved and the terminals refit once, not on every pointer move.
+ * `kept` panes (another workspace's live sessions) are hidden children of
+ * the SAME list, keyed by `keyOf`, so a pane moves between shown and kept
+ * without remounting — a workspace switch never re-attaches its terminals.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
@@ -365,8 +368,14 @@ export function PaneGrid({
   onNewSplit,
   stateOf,
   onStop,
+  keyOf = (slot) => slot,
+  kept = [],
 }: {
   layout: PaneLayout;
+  /** A pane's React key: unique across `mounted` and `kept` (a `paneKey`). */
+  keyOf?: (slot: number) => number;
+  /** Panes kept mounted, always hidden — never in the layout. */
+  kept?: { key: number; node: ReactNode }[];
   /** Stops the session in `slot` (the header's X). */
   onStop?: (slot: number) => void;
   /** A live AI run's state for its header dot (none for a shell). */
@@ -490,13 +499,25 @@ export function PaneGrid({
         onChange((l) => applyDrop(l, src, z.slot, z.zone));
       }}
     >
-      {mounted.map((slot) => {
+      {[
+        ...mounted.map((slot) => ({ key: keyOf(slot), slot, node: null as ReactNode })),
+        ...kept.map((k) => ({ key: k.key, slot: null, node: k.node })),
+      ].map(({ key, slot, node }) => {
+        if (slot === null) {
+          // The same element shape as a hidden pane below, so React keeps it.
+          return (
+            <div key={key} data-testid={`pane-kept-${key}`} className="hidden">
+              {false}
+              <div className="flex min-h-0 flex-1 flex-col">{node}</div>
+            </div>
+          );
+        }
         const r = rectOf.get(slot);
         const run = runOf(slot);
         const focused = layout.focus === slot;
         return (
           <div
-            key={slot}
+            key={key}
             data-testid={`pane-${slot}`}
             data-focused={r && focused ? "true" : undefined}
             // A menu opened in this pane stays inside it (`menuRoom`).

client/src/pages/Workspace.tsx

@@ -148,6 +148,9 @@ const CHANGES_DRAGGED_KEY = "agenthub.workspace.changesDragged";
 const CONTEXT_OPEN_KEY = "agenthub.workspace.contextOpen";
 /** localStorage key for the last workspace shown, restored on a bare /workspace. */
 const SELECTED_KEY = "agenthub.workspace.selected";
+/** Workspaces left most recently whose live panes stay mounted (hidden), so
+ *  switching back to one shows its terminals as they were, with no re-attach. */
+const KEEP_LANES = 4;
 
 /** The Changes panel's two preset widths. Its header has two controls, like a
  *  window's: Full screen (Restore once at full screen, back to the last
@@ -198,7 +201,10 @@ function slotOf(runId: string | null | undefined): number {
  *  (Settings → Appearance) the tabs hold EVERY workspace's live runs, stored
  *  once (`all`): a leaf is a `paneKey` (lane × 10 + slot), each pane is bound
  *  to its own run's lane, and focusing one selects its workspace (the URL's
- *  `?lane=` follows `?run=`). */
+ *  `?lane=` follows `?run=`). Without it, the live panes of the last
+ *  `KEEP_LANES` workspaces left stay mounted too — hidden siblings in the same
+ *  grid, each keyed by its `paneKey` — so a workspace switch neither disposes
+ *  nor re-attaches a terminal. */
 function ConsoleArea({
   lanes,
   selectedLaneId,
@@ -481,6 +487,51 @@ function ConsoleArea({
   const mounted = [...new Set([...shown, ...tabs.map((r) => r.slot ?? 0)])].sort((a, b) => a - b);
   const focusSlot = view.focus;
 
+  // The workspaces just left, newest first — updated DURING the render that
+  // switches, because the switch's own commit would otherwise unmount the
+  // panes before an effect could keep them. Idempotent on a repeat render.
+  const leftLanes = useRef<number[]>([]);
+  const lastLane = useRef(selectedLaneId);
+  if (lastLane.current !== selectedLaneId) {
+    const prev = lastLane.current;
+    if (prev !== null)
+      leftLanes.current = [prev, ...leftLanes.current.filter((id) => id !== prev)].slice(
+        0,
+        KEEP_LANES
+      );
+    lastLane.current = selectedLaneId;
+  }
+  const kept = allTabs
+    ? []
+    : (activeRuns?.items ?? [])
+        .filter(
+          (r) =>
+            r.laneId !== null &&
+            r.laneId !== selectedLaneId &&
+            r.status === "running" &&
+            leftLanes.current.includes(r.laneId)
+        )
+        .map((r) => {
+          const id = r.laneId as number;
+          const slot = r.slot ?? 0;
+          return {
+            key: paneKey(id, slot),
+            node: (
+              <LaneConsolePane
+                lanes={lanes}
+                laneId={id}
+                slot={slot}
+                binaryStatus={binaryStatus}
+                activeRuns={activeRuns}
+                onNewSession={onNewSession}
+                onRunExit={onRunsChanged}
+                autoFocus={false}
+                attachRequest={null}
+              />
+            ),
+          };
+        });
+
   // Alt+1–4 focus a pane, Alt+\ splits the focused one right with a new
   // terminal, Alt+W takes it out into a tab of its own. Session view only, and never from inside a
   // terminal or a field, whose keys are their own (Ctrl+M lets Tab out of a
@@ -578,6 +629,9 @@ function ConsoleArea({
           onNewSplit={(target, edge, tool) => void newSplit(target, edge, tool)}
           stateOf={stateOf}
           onStop={(slot) => void stopPane(slot)}
+          // Keyed by workspace too, so a kept pane coming back is the same one.
+          keyOf={allTabs ? undefined : (slot) => paneKey(selectedLaneId ?? 0, slot)}
+          kept={kept}
           renderPane={(slot) => (
             <LaneConsolePane
               lanes={lanes}

client/src/pages/__tests__/Workspace.test.tsx

@@ -675,6 +675,56 @@ describe("console tabs", () => {
     expect(screen.getByTestId("console-tab-new")).toBeDisabled();
   });
 
+  // Switching workspaces used to hand slot 0 of the new workspace the SAME
+  // pane as the old one's slot 0: its terminal was disposed and a new one
+  // attached, replaying the whole screen — the lag of every switch.
+  it("keeps a workspace's terminals mounted (hidden) while another workspace is shown", async () => {
+    withRuns([live(0), { ...live(0), id: "agenthub-lane-2", laneId: 2, slot: 0 }]);
+    function Go() {
+      const [, setParams] = useSearchParams();
+      return (
+        <>
+          {[1, 2].map((id) => (
+            <button
+              key={id}
+              data-testid={`go-${id}`}
+              onClick={() => setParams({ lane: String(id), run: `agenthub-lane-${id}` })}
+            />
+          ))}
+        </>
+      );
+    }
+    render(
+      <MemoryRouter initialEntries={["/workspace?lane=1&run=agenthub-lane-1"]}>
+        <Go />
+        <Workspace />
+      </MemoryRouter>
+    );
+    await settle();
+    await settle();
+    const term = (id: string) =>
+      screen.getAllByTestId("terminal-view").find((el) => el.dataset.runId === id);
+    const first = term("agenthub-lane-1");
+    expect(first).toBeDefined();
+    const user = userEvent.setup();
+    await user.click(screen.getByTestId("go-2"));
+    await settle();
+    const second = term("agenthub-lane-2");
+    expect(second?.closest(".hidden")).toBeNull();
+    // Lane 1's terminal is the same element, only hidden.
+    expect(term("agenthub-lane-1")).toBe(first);
+    expect(first!.closest(".hidden")).not.toBeNull();
+    await user.click(screen.getByTestId("go-1"));
+    await settle();
+    expect(term("agenthub-lane-1")).toBe(first);
+    expect(first!.closest(".hidden")).toBeNull();
+    expect(term("agenthub-lane-2")).toBe(second);
+    // Only the shown workspace's tabs are in the tab bar.
+    expect(
+      screen.getAllByRole("tab").filter((t) => t.dataset.testid?.startsWith("console-tab-"))
+    ).toHaveLength(1);
+  });
+
   it("with tabs from every workspace on, shows another workspace's session and selects it on click", async () => {
     window.localStorage.setItem("agenthub.workspace.allTabs", "1");
     withRuns([live(0), { ...live(0), id: "agenthub-lane-2-1", laneId: 2, slot: 1 }]);