perf(search): switch editors in the frame the Switcher closes

Releasing Ctrl in the Switcher (and picking from Recent Files / Recent
Locations) closed the popup, painted a frame of the old editor, and only
then opened the selection from a timeout. The popup now unmounts
synchronously (`flushSync`), gives focus back, and the selection opens in
the same task, so closing and switching paint together.

`pnpm perf` measures Ctrl+Tab between file tabs (`tabs.switcher*`) with a
p50 budget of two frames, and the agent tab switch budget is tightened.

perf/budgets.json

@@ -72,10 +72,11 @@
 
     "tabs.openSmallMs": 100,
     "tabs.switchP95Ms": 33,
+    "tabs.switcherP50Ms": 33,
 
     "agentTabs.conversationNodes": 20000,
     "agentTabs.rowsAlwaysRendered": 0,
-    "agentTabs.switchP95Ms": 66,
+    "agentTabs.switchP95Ms": 50,
     "agentTabs.switchMainThreadMs": 50,
     "agentTabs.switchTbtMs": 50,
     "agentTabs.scrollJumps": 0,

perf/scenarios.ts

@@ -455,7 +455,21 @@ const tabs: Scenario = {
         switches.push(await h.measure(() => tab.click(), ".cm-content", `// tab-${i}`));
       }
     }
-    return { openSmallMs, ...latencySummary(switches, "switch") };
+    // The Switcher (Ctrl+Tab) back and forth between the two most recent tabs.
+    const before = await h.counters();
+    const switcher: number[] = [];
+    for (let i = 0; i < 10; i++) {
+      const tag = i % 2 ? names.length - 1 : names.length - 2;
+      switcher.push(await h.measure(() => h.pressAction("navigate.switcher"), ".cm-content", `// tab-${tag}`));
+      await h.page.waitForTimeout(150);
+    }
+    const work = Harness.delta(before, await h.counters());
+    return {
+      openSmallMs,
+      ...latencySummary(switches, "switch"),
+      ...latencySummary(switcher, "switcher"),
+      switcherMainThreadMs: work.taskMs / switcher.length,
+    };
   },
 };
 

src/features/search/__tests__/dialogs.test.tsx

@@ -55,7 +55,7 @@ vi.mock("../find/windowStore", async (orig) => ({
 const contributions = await import("@/workbench/contributions");
 const { commands } = await import("@/platform/ipc");
 const { RecentPopupsOverlay } = await import("../recent/RecentPopups");
-const { showRecentPopup, hideRecentPopup, useRecentPopup } = await import("../recent/store");
+const { cycleSwitcher, showRecentPopup, hideRecentPopup, useRecentPopup } = await import("../recent/store");
 const { openLocation } = await import("../common/open");
 const { usePlatform } = await import("@/platform/platformInfo");
 const { allActions } = await import("@/platform/actions");
@@ -476,6 +476,42 @@ describe("Recent Files multi-selection", () => {
   });
 });
 
+describe("Switcher", () => {
+  afterEach(() => vi.restoreAllMocks());
+
+  it("opens the selection when Ctrl is released, once the popup gave focus back, in the same task", async () => {
+    const tab = (key: string, lastActive: number) => ({
+      input: { kind: "agent-session", key },
+      title: key,
+      icon: "toolwindow-agents",
+      lastActive,
+    });
+    const seen: Array<{ key: string; popup: boolean; focus: Element | null }> = [];
+    vi.spyOn(commands, "recentFiles").mockResolvedValue({ status: "ok", data: [] });
+    vi.spyOn(contributions, "maybeEditorService").mockReturnValue({
+      openTabs: () => [tab("task-b", 2), tab("task-a", 1)],
+      openPaths: () => [],
+      activePath: () => null,
+      openInput: async (input: { key: string }) => {
+        seen.push({ key: input.key, popup: !!screen.queryByRole("dialog"), focus: document.activeElement });
+      },
+    } as unknown as import("@/workbench/contributions").EditorService);
+    render(
+      <>
+        <button type="button">editor</button>
+        <RecentPopupsOverlay />
+      </>,
+    );
+    const editor = screen.getByRole("button", { name: "editor" });
+    editor.focus();
+    act(() => cycleSwitcher(1));
+    await tick();
+    expect(screen.getByRole("dialog", { name: "Switcher" })).toBeInTheDocument();
+    act(() => void fireEvent.keyUp(window, { key: "Control" }));
+    expect(seen).toEqual([{ key: "task-a", popup: false, focus: editor }]);
+  });
+});
+
 describe("Find in Files dialog", () => {
   it("streams results and highlights the matches while replacing, as IDEA", async () => {
     render(<FindInFilesOverlay />);

src/features/search/recent/RecentPopups.tsx

@@ -1,5 +1,6 @@
 import { useFilePresentation } from "@/platform/filePresentation";
 import { useEffect, useMemo, useRef, useState, type KeyboardEvent, type MouseEvent as ReactMouseEvent } from "react";
+import { flushSync } from "react-dom";
 import clsx from "clsx";
 import { editorsFor, getNavigationHistory } from "@/editor/api";
 import { call, commands, type OutlineItem } from "@/platform/ipc";
@@ -61,10 +62,13 @@ export function RecentPopupsOverlay() {
 
 const NAV_KEYS = ["ArrowDown", "ArrowUp", "PageDown", "PageUp"];
 
-/** Defers work until the dialog unmounted and restored focus. */
+/**
+ * Runs `fn` once the dialog has unmounted and given focus back, in the same frame: the
+ * popup closing and what it opens paint together, without a frame of the old editor between.
+ */
 const afterClose = (fn: () => void) => {
-  hideRecentPopup();
-  setTimeout(fn, 0);
+  flushSync(hideRecentPopup);
+  fn();
 };
 
 // ---------------------------------------------------------------------------