Expand workspace panel actions and keyboard navigation (#3685)

* fix(app): keep file navigation in place

Retargeting a File tab allocated a new tab identity, causing the tab track to unmount and remount during navigation. Preserve the identity for File-to-File changes while retaining new instances for cross-kind replacements.

* feat(app): expand workspace panel actions

Keep the Command Center aligned with workspace tab and pane primitives. Add explicit panel placement, focused open behavior, keyboard navigation for empty panes, and a Changes pill that toggles the visible Side panel without closing its tab.

packages/app/e2e/browser/empty-pane-keyboard.spec.ts

@@ -0,0 +1,79 @@
+import { expect, type Locator, type Page } from "@playwright/test";
+import { test } from "../support/fixtures";
+import { runWorkspaceActionFromCommandCenter } from "../support/helpers/command-center-workspace-actions";
+import { gotoWorkspace } from "../support/helpers/launcher";
+import { seedWorkspace } from "../support/helpers/seed-client";
+import { waitForWorkspaceTabsVisible } from "../support/helpers/workspace-tabs";
+
+function visibleEmptyPane(page: Page): Locator {
+  return page.getByTestId("workspace-pane-empty-state").filter({ visible: true });
+}
+
+async function expectLauncherSelection(launcher: Locator, name: string): Promise<void> {
+  await expect(launcher.getByRole("button", { name })).toBeFocused();
+}
+
+async function openSidePanelWithKeyboard(page: Page): Promise<Locator> {
+  const modifier = process.platform === "darwin" ? "Meta" : "Control";
+  await page.keyboard.press(`${modifier}+E`);
+  const launcher = page
+    .getByTestId("workspace-side-panel")
+    .filter({ visible: true })
+    .getByTestId("workspace-pane-empty-state");
+  await expect(launcher).toBeVisible({ timeout: 30_000 });
+  return launcher;
+}
+
+test.describe("Empty pane keyboard launcher", () => {
+  test("Cmd+E focuses the launcher and Enter opens the selected view", async ({ page }) => {
+    const workspace = await seedWorkspace({ repoPrefix: "empty-pane-side-panel-keyboard-" });
+
+    try {
+      await gotoWorkspace(page, workspace.workspaceId);
+      await waitForWorkspaceTabsVisible(page);
+
+      const launcher = await openSidePanelWithKeyboard(page);
+      await expectLauncherSelection(launcher, "Changes");
+
+      await page.keyboard.press("ArrowDown");
+      await expectLauncherSelection(launcher, "Files");
+      await page.keyboard.press("Enter");
+
+      await expect(
+        page
+          .getByTestId("workspace-side-panel")
+          .filter({ visible: true })
+          .getByTestId("workspace-tab-files"),
+      ).toBeVisible({ timeout: 30_000 });
+    } finally {
+      await workspace.cleanup();
+    }
+  });
+
+  test("a new split pane focuses its launcher and wraps arrow navigation", async ({ page }) => {
+    const workspace = await seedWorkspace({ repoPrefix: "empty-pane-split-keyboard-" });
+
+    try {
+      await gotoWorkspace(page, workspace.workspaceId);
+      await waitForWorkspaceTabsVisible(page);
+      await runWorkspaceActionFromCommandCenter(page, "Split pane right");
+
+      const launcher = visibleEmptyPane(page);
+      await expect(launcher).toBeVisible({ timeout: 30_000 });
+      await expectLauncherSelection(launcher, "Changes");
+
+      await page.keyboard.press("ArrowUp");
+      await expectLauncherSelection(launcher, "Terminal");
+      await page.keyboard.press("ArrowDown");
+      await expectLauncherSelection(launcher, "Changes");
+      await page.keyboard.press("ArrowDown");
+      await page.keyboard.press("Enter");
+
+      await expect(page.getByTestId("workspace-tab-files").filter({ visible: true })).toBeVisible({
+        timeout: 30_000,
+      });
+    } finally {
+      await workspace.cleanup();
+    }
+  });
+});