fix(app): stabilize diff reviews and workspace tabs

Reuse measured diff geometry when review controls open, keeping the interaction responsive on large changes. Preserve complete tab-track snapshots while exact content widths settle so new tabs and retained workspaces do not flash or truncate.

packages/app/e2e/browser/diff-performance.spec.ts

@@ -25,6 +25,39 @@ const MAX_MEDIAN_FRAME_MS = POC_MEDIAN_FRAME_MS * 1.1;
 const MAX_P95_FRAME_MS = POC_P95_FRAME_MS * 1.1;
 
 diffPerfDescribe("Diff canvas performance", () => {
+  test("opening a review stays responsive in a large diff", async ({ page }) => {
+    test.setTimeout(120_000);
+    const repo = await createLargeDiffRepository();
+    const client = await connectSeedClient();
+    let cdp: CDPSession | null = null;
+
+    try {
+      const created = await client.createWorkspace({
+        source: { kind: "directory", path: repo.path },
+      });
+      if (!created.workspace) {
+        throw new Error(created.error ?? "Failed to create diff performance workspace");
+      }
+
+      await configureUnwrappedUnifiedDiff(page);
+      await page.setViewportSize({ width: 1400, height: 900 });
+      await page.goto(buildHostWorkspaceRoute(getServerId(), created.workspace.id));
+      await waitForWorkspaceTabsVisible(page);
+      await openChangesPanel(page);
+      await expect(page.getByTestId("git-diff-canvas")).toBeVisible({ timeout: 30_000 });
+
+      cdp = await page.context().newCDPSession(page);
+      await cdp.send("Emulation.setCPUThrottlingRate", { rate: CPU_SLOWDOWN });
+      const elapsedMs = await openFirstReviewEditor(page);
+      expect(elapsedMs).toBeLessThanOrEqual(200 * CPU_SLOWDOWN);
+    } finally {
+      await cdp?.send("Emulation.setCPUThrottlingRate", { rate: 1 }).catch(() => undefined);
+      await cdp?.detach().catch(() => undefined);
+      await client.close().catch(() => undefined);
+      await repo.cleanup().catch(() => undefined);
+    }
+  });
+
   test("one canvas stays filled through continuous fast scrolling", async ({ page }, testInfo) => {
     test.setTimeout(180_000);
     const repo = await createLargeDiffRepository();
@@ -241,6 +274,21 @@ async function configureUnwrappedUnifiedDiff(page: Page): Promise<void> {
   );
 }
 
+async function openFirstReviewEditor(page: Page): Promise<number> {
+  const body = page.getByTestId("diff-file-0-body");
+  const canvas = page.getByTestId("git-diff-canvas");
+  const [bodyBounds, fontSize] = await Promise.all([
+    body.boundingBox(),
+    canvas.evaluate((element) => Number.parseFloat(getComputedStyle(element).fontSize)),
+  ]);
+  if (!bodyBounds) throw new Error("Expanded diff body has no bounds");
+  const lineHeight = Math.round(fontSize * 1.5);
+  const startedAt = performance.now();
+  await page.mouse.click(bodyBounds.x + 120, bodyBounds.y + lineHeight * 1.5);
+  await expect(page.getByTestId("inline-review-editor")).toBeVisible();
+  return performance.now() - startedAt;
+}
+
 async function expectSingleCanvasSurface(page: Page): Promise<void> {
   await expect(page.locator("canvas")).toHaveCount(1);
   await expect(page.locator('[data-testid^="diff-code-row-"]')).toHaveCount(0);

packages/app/e2e/browser/launcher-tab.spec.ts

@@ -12,6 +12,7 @@ import {
   waitForTabWithTitle,
   measureTileTransition,
   sampleTabsDuringTransition,
+  expectTabTitleFits,
   terminalSurfaceLocator,
 } from "../support/helpers/launcher";
 import { expectComposerVisible, composerLocator } from "../support/helpers/composer";
@@ -197,7 +198,7 @@ test.describe("Tab transitions (no flash)", () => {
     await gotoWorkspace(page, workspace.workspaceId);
 
     // Sample tabs at high frequency across the transition
-    const snapshots = await sampleTabsDuringTransition(page, () => clickNewChat(page), 2_000, 30);
+    const snapshots = await sampleTabsDuringTransition(page, () => clickNewChat(page));
 
     // Every snapshot should have at least one tab — no blank/zero-tab frames
     for (const snapshot of snapshots) {
@@ -207,11 +208,15 @@ test.describe("Tab transitions (no flash)", () => {
     // Tab count should never spike excessively (no duplicate flash from add-then-remove).
     // When running in-suite, previous tests may have created tabs on the shared workspace,
     // so we allow +2 tolerance for accumulated state and React render batching.
-    const counts = snapshots.map((s) => s.length);
+    const counts = snapshots.map((snapshot) => snapshot.length);
     const maxCount = Math.max(...counts);
     const initialCount = counts[0] ?? 0;
 
     expect(maxCount).toBeLessThanOrEqual(initialCount + 2);
+    expect(new Set(snapshots.map((snapshot) => JSON.stringify(snapshot))).size).toBeLessThanOrEqual(
+      2,
+    );
+    await expectTabTitleFits(page, "New Agent", { min: 96, max: 160 });
   });
 
   test("Terminal transition completes within visual budget", async ({ page }) => {