perf(agents): switch back to a long conversation without rebuilding it

Agent task tabs now stay mounted while another tab is active (the five most
recently used per pane). A hidden one skips rendering with
content-visibility: hidden, which keeps its layout and scroll offset, so
showing it again costs a few milliseconds instead of building and laying out
the whole conversation. Hidden tabs don't mark their task read and close Find.

Thinking blocks and tool output now sit in a wrapper that skips off-screen
layout like other entries; on <details> itself WebKit wouldn't expand them,
so they were laid out in full on every resize and reopen.

src/editor/core/view/EditorArea.module.css

@@ -30,6 +30,19 @@
   min-height: 0;
   min-width: 0;
   position: relative;
+  isolation: isolate;
+}
+/*
+ * Kept-alive tabs share the content box; hiding one only skips its rendering, not its layout.
+ * Nothing inherited changes (pointer-events, inert): that would restyle the whole conversation.
+ */
+.kept {
+  position: absolute;
+  inset: 0;
+}
+.keptHidden {
+  content-visibility: hidden;
+  z-index: -1;
 }
 .host {
   flex: 1;

src/editor/core/view/EditorArea.tsx

@@ -19,11 +19,13 @@ import {
   getFileDecoration,
   prepareMenuGroups,
   textEditorWrapperFor,
+  EditorShownProvider,
   useContributions,
   useFileDecorations,
   useEditorDocumentRevision,
   useTextEditorWrappers,
   type EditorInput,
+  type EditorKindContribution,
 } from "@/workbench/contributions";
 import { useDistractionFree } from "@/workbench/distractionFree";
 import { EmptyEditor } from "@/workbench/EditorArea";
@@ -276,6 +278,7 @@ function PaneView({ paneId }: { paneId: string }) {
 
   if (!pane) return null;
   const active = pane.tabs.find((t) => t.id === pane.active) ?? null;
+  const kept = keptTabs(pane.tabs, pane.mru, kinds);
 
   const onFocus = () => {
     setContext("editorFocus", true);
@@ -355,7 +358,14 @@ function PaneView({ paneId }: { paneId: string }) {
         aria-labelledby={active ? `editor-${paneId}-tab-${active.id}` : undefined}
         ref={contentRef}
       >
-        {active ? <PaneContent paneId={paneId} tab={active} tabsVisible={!hidden} /> : <EmptyEditor />}
+        {kept.map((tab) => (
+          <KindEditor key={tab.id} tab={tab} shown={tab === active} kept />
+        ))}
+        {!active ? (
+          <EmptyEditor />
+        ) : (
+          !kept.includes(active) && <PaneContent paneId={paneId} tab={active} tabsVisible={!hidden} />
+        )}
         {dropZone && <div className={clsx(styles.dropZone, styles[`drop-${dropZone}`])} aria-hidden="true" />}
       </div>
     </section>
@@ -370,7 +380,6 @@ function virtualTooltip(input: EditorInput): string | null {
 }
 
 function PaneContent({ paneId, tab, tabsVisible }: { paneId: string; tab: Tab; tabsVisible: boolean }) {
-  const kinds = useContributions((s) => s.editorKinds);
   const crumbs = !useDistractionFree((s) => s.active);
   const wrappers = useTextEditorWrappers((s) => s.wrappers);
   if (isTextInput(tab.input)) {
@@ -402,12 +411,41 @@ function PaneContent({ paneId, tab, tabsVisible }: { paneId: string; tab: Tab; t
       </>
     );
   }
+  return <KindEditor key={tab.id} tab={tab} shown />;
+}
+
+/** Tabs of a pane kept mounted while hidden: the most recently used ones of `keepAlive` kinds. */
+const KEPT_TABS = 5;
+
+function keptTabs(tabs: Tab[], mru: string[], kinds: Map<string, EditorKindContribution>): Tab[] {
+  const recent = mru
+    .map((id) => tabs.find((t) => t.id === id))
+    .filter((t): t is Tab => !!t && !!kinds.get(t.input.kind)?.keepAlive)
+    .slice(0, KEPT_TABS);
+  // In tab order, so switching tabs doesn't move their DOM (which would lose scroll offsets).
+  return tabs.filter((t) => recent.includes(t));
+}
+
+/**
+ * `shown`: the pane's visible tab, which focusing the editor area reaches. A hidden `kept` tab
+ * keeps its layout (content-visibility), so showing it again needn't lay it out from scratch;
+ * its content can't be focused or hit while skipped.
+ */
+function KindEditor({ tab, shown, kept = false }: { tab: Tab; shown: boolean; kept?: boolean }) {
+  const kinds = useContributions((s) => s.editorKinds);
   const kind = kinds.get(tab.input.kind);
   if (!kind) return <EmptyState icon="file" title={tx`No editor for “${tab.input.kind}”`} />;
   const Component = kind.component;
   return (
-    <div className={styles.kind} data-editor-focus="" tabIndex={-1} key={tab.id}>
-      <Component input={tab.input} />
+    <div
+      className={clsx(styles.kind, kept && styles.kept, !shown && styles.keptHidden)}
+      data-editor-focus={shown ? "" : undefined}
+      tabIndex={-1}
+      aria-hidden={shown ? undefined : true}
+    >
+      <EditorShownProvider value={shown}>
+        <Component input={tab.input} />
+      </EditorShownProvider>
     </div>
   );
 }

src/editor/core/view/keptTabs.test.tsx

@@ -0,0 +1,98 @@
+import { afterAll, afterEach, describe, expect, it, vi } from "vitest";
+import { act, cleanup, render, screen } from "@testing-library/react";
+import { useState } from "react";
+import { registerEditorKind, type EditorInput } from "@/workbench/contributions";
+import { useTabs } from "../controller";
+import type { Tab } from "../model/tabs";
+import { EditorAreaView } from "./EditorArea";
+
+vi.mock("../controller", async (original) => ({
+  ...(await original<object>()),
+  editorController: () => ({ publishContext: vi.fn(), tabMenu: () => [] }),
+}));
+vi.mock("./TextEditorHost", () => ({
+  TextEditorHost: () => <input aria-label="Source" />,
+}));
+vi.mock("./FindBar", () => ({ FindBar: () => null }));
+vi.mock("./Banner", () => ({
+  EditorBanners: () => null,
+  VirtualBanner: () => null,
+}));
+vi.mock("./StatusItems", () => ({ CodeBreadcrumbs: () => null }));
+
+const mounts: string[] = [];
+function Kept({ input }: { input: EditorInput }) {
+  const name = input.key;
+  useState(() => mounts.push(name));
+  return <div data-testid={name}>{name}</div>;
+}
+const disposers = [
+  registerEditorKind({
+    kind: "kept",
+    component: Kept,
+    title: (i) => i.key,
+    keepAlive: true,
+  }),
+  registerEditorKind({ kind: "plain", component: Kept, title: (i) => i.key }),
+];
+
+/** Shown, or kept mounted behind the shown tab (its rendering skipped). */
+const shown = (id: string) => !screen.getByTestId(id).closest("[aria-hidden]");
+
+const tab = (id: string, kind: string): Tab => ({
+  id,
+  input: { kind, key: id },
+  preview: false,
+  pinned: false,
+});
+const original = useTabs.getState();
+
+function show(tabs: Tab[], active: string) {
+  act(() =>
+    useTabs.setState((s) => {
+      const mru = [active, ...(s.panes.p1?.mru ?? []).filter((id) => id !== active && tabs.some((t) => t.id === id))];
+      return {
+        layout: { kind: "pane", paneId: "p1" },
+        panes: { p1: { id: "p1", tabs, active, mru } },
+        activePane: "p1",
+        closed: [],
+      };
+    }),
+  );
+}
+
+afterEach(() => {
+  cleanup();
+  mounts.length = 0;
+  useTabs.setState(original, true);
+});
+afterAll(() => disposers.forEach((d) => d()));
+
+describe("keep-alive editor kinds", () => {
+  it("keep their recent tabs mounted and hidden while another tab is active", () => {
+    const tabs = [tab("a", "kept"), tab("b", "kept"), tab("c", "plain")];
+    show(tabs, "a");
+    render(<EditorAreaView />);
+    show(tabs, "b");
+    show(tabs, "c");
+    expect(shown("a")).toBe(false);
+    expect(shown("b")).toBe(false);
+    expect(shown("c")).toBe(true);
+    show(tabs, "a");
+    show(tabs, "c");
+    // Only the plain tab is built again; the kept ones are shown as they were.
+    expect(mounts).toEqual(["a", "b", "c", "c"]);
+    expect(screen.queryAllByTestId("c")).toHaveLength(1);
+  });
+
+  it("let the least recently used kept tabs go beyond the limit", () => {
+    const tabs = Array.from({ length: 7 }, (_, i) => tab(`k${i}`, "kept"));
+    show(tabs, "k0");
+    render(<EditorAreaView />);
+    for (const t of tabs) show(tabs, t.id);
+    expect(screen.queryByTestId("k0")).toBeNull();
+    expect(screen.queryByTestId("k1")).toBeNull();
+    expect(shown("k2")).toBe(false);
+    expect(shown("k6")).toBe(true);
+  });
+});

src/features/agents/Session.module.css

@@ -157,7 +157,10 @@
 .stream:focus {
   outline: none;
 }
-/* Skip off-screen output layout; editable messages must repaint immediately when focus/content changes. */
+/*
+ * Skip off-screen output layout; editable messages must repaint immediately when focus/content changes.
+ * Thinking and tool output sit in a <div>: on <details> itself WebKit wouldn't expand them.
+ */
 .stream > :not(.message):not(details) {
   content-visibility: auto;
   contain-intrinsic-block-size: auto 80px;

src/features/agents/SessionEditor.tsx

@@ -1,7 +1,9 @@
 /** An agent task as an editor tab: the conversation, a composer and the task's changed files. */
 import {
   Fragment,
+  memo,
   startTransition,
+  useCallback,
   useEffect,
   useLayoutEffect,
   useMemo,
@@ -15,7 +17,7 @@ import type { AgentChange, AgentEntry, AgentImage, AgentLimit, AgentRetry, Agent
 import { executeAction } from "@/platform/actions";
 import { call, commands, onIdeEvent } from "@/platform/ipc";
 import { basename, relativeDir, useProject } from "@/platform/project";
-import { editorService, notifications, type EditorInput } from "@/workbench/contributions";
+import { editorService, notifications, useEditorShown, type EditorInput } from "@/workbench/contributions";
 import {
   Button,
   EmptyState,
@@ -105,15 +107,17 @@ function TaskSession({ id, docked }: { id: string; docked: boolean }) {
     if (!useAgents.getState().entries.has(id)) void loadEntries(id).catch(() => undefined);
     void refreshChanges(id);
   }, [id]);
+  // A kept-alive tab stays mounted behind the shown one: it isn't read there, nor searched.
+  const shown = useEditorShown();
   useEffect(() => {
-    if (!docked || !changesOpen) return markVisible(id);
-  }, [id, docked, changesOpen]);
+    if (shown && (!docked || !changesOpen)) return markVisible(id);
+  }, [id, docked, changesOpen, shown]);
   useEffect(() => {
     if (view && !useAgents.getState().entries.has(entriesKey(id, view))) {
       void loadEntries(id, view).catch(() => undefined);
     }
   }, [id, view]);
-  const openSubagent = (sub: string) => setPath((p) => [...p, sub]);
+  const openSubagent = useCallback((sub: string) => setPath((p) => [...p, sub]), []);
   const [finding, setFinding] = useState(false);
   const [findFocus, setFindFocus] = useState(0);
   const [session] = useState<FocusedSession>(() => ({
@@ -157,7 +161,7 @@ function TaskSession({ id, docked }: { id: string; docked: boolean }) {
           entries={viewEntries ?? null}
           subagent={view ? (cards.at(-1) ?? null) : null}
           onOpenSubagent={openSubagent}
-          finding={finding}
+          finding={finding && shown}
           findFocus={findFocus}
           onCloseFind={() => setFinding(false)}
         />
@@ -441,9 +445,11 @@ function Stream({ task, entries, subagent, onOpenSubagent, finding, findFocus, o
   useEffect(() => {
     if (start > 0) startTransition(() => setShown((n) => n + ENTRY_BATCH));
   }, [start]);
+  // Hidden behind another tab, it has no layout to scroll; shown again, it follows the end again.
+  const visible = useEditorShown();
   useLayoutEffect(() => {
     const el = ref.current;
-    if (!el || entries === null) return;
+    if (!el || entries === null || !visible) return;
     if (position.current.stick) el.scrollTop = el.scrollHeight;
     else if (!restored.current) el.scrollTop = position.current.top;
     restored.current = true;
@@ -726,7 +732,8 @@ const TOOL_ICONS: Record<string, string> = {
   think: "todo",
 };
 
-function Entry({
+/** Memoized: the earlier entries rendered behind a reopened conversation leave the rest alone. */
+const Entry = memo(function Entry({
   task,
   entry,
   turn,
@@ -747,12 +754,14 @@ function Entry({
       return <Markdown text={entry.text} cwd={task.cwd} />;
     case "thought":
       return (
-        <details className={styles.thought}>
-          <summary>
-            <T>Thinking</T>
-          </summary>
-          <div className={styles.thoughtText}>{entry.text}</div>
-        </details>
+        <div>
+          <details className={styles.thought}>
+            <summary>
+              <T>Thinking</T>
+            </summary>
+            <div className={styles.thoughtText}>{entry.text}</div>
+          </details>
+        </div>
       );
     case "tool":
       return <ToolEntry task={task} entry={entry} />;
@@ -801,7 +810,7 @@ function Entry({
         </div>
       );
   }
-}
+});
 
 function UserMessage({
   task,
@@ -1049,10 +1058,12 @@ function ToolEntry({ task, entry }: { task: AgentTask; entry: Extract<AgentEntry
   );
   if (!entry.output) return <div className={styles.tool}>{head}</div>;
   return (
-    <details className={styles.toolDetails}>
-      <summary className={styles.tool}>{head}</summary>
-      <pre className={styles.output}>{entry.output}</pre>
-    </details>
+    <div>
+      <details className={styles.toolDetails}>
+        <summary className={styles.tool}>{head}</summary>
+        <pre className={styles.output}>{entry.output}</pre>
+      </details>
+    </div>
   );
 }
 

src/features/agents/index.ts

@@ -73,6 +73,8 @@ export function activate(): () => void {
       icon: () => "toolwindow-agents",
       // Tabs of tasks deleted meanwhile are closed when the tasks load (`connect`).
       restorable: true,
+      // Rebuilding a long conversation on every switch back is slow.
+      keepAlive: true,
     }),
     registerEditorKind({
       kind: HOME_KIND,

src/features/agents/sessionState.test.tsx

@@ -2,6 +2,7 @@ import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { act, fireEvent, render, screen, waitFor, within } from "@testing-library/react";
 import { registerActions } from "@/platform/actions";
 import { useProject } from "@/platform/project";
+import { EditorShownProvider } from "@/workbench/contributions";
 import type { AgentChange, AgentEntry, AgentTask } from "@/bindings";
 import { AgentsView } from "./AgentsView";
 import { SessionEditor } from "./SessionEditor";
@@ -179,6 +180,20 @@ describe("a pending permission request in the conversation", () => {
 });
 
 describe("session state across tasks and hosts", () => {
+  it("leaves a task unread while its kept-alive tab is hidden, and reads it once shown", () => {
+    vi.spyOn(document, "hasFocus").mockReturnValue(true);
+    useAgents.setState({ unread: new Set(["A"]) });
+    const session = (shown: boolean) => (
+      <EditorShownProvider value={shown}>
+        <SessionEditor input={input("A")} />
+      </EditorShownProvider>
+    );
+    const { rerender } = render(session(false));
+    expect(useAgents.getState().unread.has("A")).toBe(true);
+    rerender(session(true));
+    expect(useAgents.getState().unread.has("A")).toBe(false);
+  });
+
   it("fetches a conversation once and shows it again from the store when its tab reopens", async () => {
     const entries = new Map(useAgents.getState().entries);
     entries.delete("A");

src/workbench/contributions.ts

@@ -122,6 +122,12 @@ export interface EditorKindContribution {
   restorable?: boolean;
   /** Can remain visible in a window without a project (for example, an external Git log). */
   standalone?: boolean;
+  /**
+   * Its recently used tabs stay mounted, hidden, while another tab is active: switching back
+   * shows them as they were instead of building them again (long agent conversations). A hidden
+   * tab keeps running; `useEditorShown` tells it whether it is the one shown.
+   */
+  keepAlive?: boolean;
   /** Last tab showing this input closed (including split tabs and tab-limit eviction). */
   onClose?(input: EditorInput): void;
   /**
@@ -318,6 +324,12 @@ export const registerWorkbenchBanner = (c: WorkbenchBannerContribution) => regis
 export const registerToolWindow = (c: ToolWindowContribution) => register("toolWindows", c.id, c);
 export const registerStatusItem = (c: StatusItemContribution) => register("statusItems", c.id, c);
 export const registerOverlay = (c: OverlayContribution) => register("overlays", c.id, c);
+
+const EditorShown = createContext(true);
+export const EditorShownProvider = EditorShown.Provider;
+/** Whether this editor tab is the one its pane shows (a `keepAlive` tab stays mounted when it isn't). */
+export const useEditorShown = () => useContext(EditorShown);
+
 export const registerEditorKind = (c: EditorKindContribution) => register("editorKinds", c.kind, c);
 export const registerTitleItem = (c: TitleItemContribution) => register("titleItems", c.id, c);
 export const registerServiceView = (c: ServiceViewContribution) => register("serviceViews", c.id, c);