fix(app): retain window dragging in compact panes

Keep the shared titlebar drag region when pane width replaces full desktop chrome. Cover two compact panes with both sidebars closed through the desktop host adapter.

Co-authored-by: Elsa Granger <zeyugao@outlook.com>

packages/app/e2e/browser/direct-terminal-shortcut-pane-placement.spec.ts

@@ -1,3 +1,4 @@
+import { installLocalDesktopHost } from "../support/helpers/local-desktop-host";
 import type { Locator, Page } from "@playwright/test";
 import { expect, test } from "../support/fixtures";
 import { runWorkspaceActionFromCommandCenter } from "../support/helpers/command-center-workspace-actions";
@@ -104,3 +105,33 @@ test("compact pane launch stays in its owning pane and pane controls remain reac
     await workspace.cleanup();
   }
 });
+
+async function expectPaneHasWindowDragSurface(pane: Locator) {
+  await expect
+    .poll(() =>
+      pane.evaluate((element) =>
+        Array.from(element.querySelectorAll<HTMLElement>("div")).some(
+          (node) => node.style.getPropertyValue("-webkit-app-region") === "drag",
+        ),
+      ),
+    )
+    .toBe(true);
+}
+
+test("compact desktop panes retain window dragging with both sidebars closed", async ({ page }) => {
+  const workspace = await seedWorkspace({ repoPrefix: "compact-pane-window-drag-" });
+  try {
+    await installLocalDesktopHost(page, workspace.repoPath);
+    const { originalPane, otherPane } = await prepareUnfocusedCompactPane(
+      page,
+      workspace.workspaceId,
+    );
+    await runWorkspaceActionFromCommandCenter(page, "Toggle left sidebar");
+    await expect(originalPane.getByTestId("workspace-tab-switcher-trigger")).toBeVisible();
+    await expect(otherPane.getByTestId("workspace-tab-switcher-trigger")).toBeVisible();
+    await expectPaneHasWindowDragSurface(originalPane);
+    await expectPaneHasWindowDragSurface(otherPane);
+  } finally {
+    await workspace.cleanup();
+  }
+});

packages/app/src/components/split-container.tsx

@@ -772,6 +772,7 @@ const SplitPaneView = memo(function SplitPaneView({
   const compactHeader = useMemo(
     () => (
       <WindowChromeSafeArea placement="inline">
+        <TitlebarDragRegion />
         {renderCompactHeader(paneTabs, activeTabDescriptor?.key ?? "", compactHeaderActions)}
       </WindowChromeSafeArea>
     ),