Guard the title bar drag region with an e2e test

The title bar renders inside its own WebContentsView and window dragging
depends on the app-region CSS staying put: a drag surface on the title and a
no-drag opt-out on every control. A refactor that drops either rule breaks
window moving or the buttons with no signal from the type checker or the unit
suite. Pin the computed declarations so that regression fails loudly.

The real OS window move cannot be driven from Playwright since synthetic mouse
events do not trigger the app-region hit-test, so this stays a CSS guard and the
actual drag plus the layered-view no-drag behavior remain manual per OS. The
focus fixes in this branch (welcome view focus in Welcome mode, background
window not stealing focus) resist a deterministic e2e assertion too: the app
never becomes OS-frontmost under the test harness, so webContents.isFocused()
always reads false and any assertion would be tautological. Those stay manual
and are written up in the PR. I worked these tests and the harness probing out
with Claude Code and confirmed the guard fails when the drag rule is removed.

Author: notluquis

test/e2e/titlebar-dragregion.test.ts

@@ -0,0 +1,71 @@
+import { expect, test } from '@playwright/test';
+import { cleanup, launchApp } from './helpers';
+
+// The title bar is drawn inside its own WebContentsView. Window dragging relies
+// on `-webkit-app-region: drag` on the `.app-title` element, and every control
+// button opts back out with `-webkit-app-region: no-drag`. Dropping either rule
+// in a refactor silently breaks window moving or makes the buttons undraggable.
+//
+// This is only a guard on the computed CSS declaration. It does NOT exercise a
+// real window move: synthetic Playwright/CDP mouse events do not drive the OS
+// drag on the app-region hit-test, so an actual drag cannot be asserted here.
+// The real window move, and the layered-WebContentsView no-drag hit-testing
+// (electron/electron#43320), still have to be verified by hand per OS.
+test('the title bar keeps its drag region and no-drag controls', async () => {
+  const { app, userDataDir, jupyterDir } = await launchApp();
+  try {
+    // Arrange: find the titlebar page. It has no window title, so it is located
+    // by the presence of its `#app-title` element, the same element-probe
+    // pattern the dialog-titlebar test uses.
+    const deadline = Date.now() + 20_000;
+    let titlebar;
+    while (!titlebar && Date.now() < deadline) {
+      for (const page of app.windows()) {
+        const hasTitle = await page
+          .evaluate(() => !!document.getElementById('app-title'))
+          .catch(() => false);
+        if (hasTitle) {
+          titlebar = page;
+          break;
+        }
+      }
+      if (!titlebar) {
+        await new Promise(resolve => setTimeout(resolve, 200));
+      }
+    }
+    if (!titlebar) {
+      throw new Error('title bar page with #app-title never appeared');
+    }
+    await titlebar.waitForLoadState('load');
+
+    // Act: read the computed app-region of the drag surface and of two control
+    // buttons. The JS-side property is `webkitAppRegion`; its value is the
+    // keyword ('drag' / 'no-drag'), not a length.
+    const regions = await titlebar.evaluate(() => {
+      const regionOf = (id: string) => {
+        const el = document.getElementById(id);
+        if (!el) {
+          return null;
+        }
+        return (getComputedStyle(el) as CSSStyleDeclaration & {
+          webkitAppRegion?: string;
+        }).webkitAppRegion;
+      };
+      return {
+        appTitle: regionOf('app-title'),
+        closeButton: regionOf('close-button'),
+        serverButton: regionOf('server-button')
+      };
+    });
+
+    // Assert: the title is draggable and the controls explicitly opt out. If a
+    // refactor drops the `-webkit-app-region` rules these flip to the inherited
+    // default ('none') and the test fails.
+    expect(regions.appTitle).toBe('drag');
+    expect(regions.closeButton).toBe('no-drag');
+    expect(regions.serverButton).toBe('no-drag');
+  } finally {
+    await app.close();
+    cleanup(userDataDir, jupyterDir);
+  }
+});