fix: sidebar auto-open retry with backoff + welcome page tests

- Replace single-attempt sidePanel.open() with autoOpenSidePanel() that
  retries up to 5 times with 500ms-5000ms backoff
- Fire on both onInstalled AND every service worker startup
- Remove misaligned arrow from welcome page, replace with text fallback
- Add 12 tests: welcome page structure, /welcome endpoint, headed launch
  navigation timing, sidebar auto-open retry logic, extension-ready event

browse/test/sidebar-ux.test.ts

@@ -1192,3 +1192,133 @@ describe('LLM-based cleanup (smart agent cleanup)', () => {
     expect(wcSrc).toContain("role') === 'navigation'");
   });
 });
+
+// ─── Welcome page + sidebar auto-open ────────────────────────────
+
+describe('welcome page', () => {
+  const welcomePath = path.join(ROOT, 'src', 'welcome.html');
+  const welcomeExists = fs.existsSync(welcomePath);
+  const welcomeSrc = welcomeExists ? fs.readFileSync(welcomePath, 'utf-8') : '';
+
+  test('welcome.html exists in browse/src/', () => {
+    expect(welcomeExists).toBe(true);
+  });
+
+  test('welcome page has GStack Browser branding', () => {
+    expect(welcomeSrc).toContain('GStack Browser');
+  });
+
+  test('welcome page has extension-ready listener to hide prompt', () => {
+    expect(welcomeSrc).toContain('gstack-extension-ready');
+    expect(welcomeSrc).toContain('sidebar-prompt');
+  });
+
+  test('welcome page does NOT have a misaligned arrow', () => {
+    // Arrow was removed because it can never align with browser chrome
+    expect(welcomeSrc).not.toContain('arrow-up');
+    expect(welcomeSrc).not.toContain('↑');
+  });
+
+  test('welcome page has left-aligned text (no center-align on headings)', () => {
+    // User preference: always left-align, never center
+    expect(welcomeSrc).not.toMatch(/text-align:\s*center/);
+  });
+
+  test('welcome page uses dark theme', () => {
+    expect(welcomeSrc).toContain('#0C0C0C'); // --base (near-black)
+    expect(welcomeSrc).toContain('#141414'); // --surface (card bg)
+  });
+});
+
+describe('server /welcome endpoint', () => {
+  const serverSrc = fs.readFileSync(path.join(ROOT, 'src', 'server.ts'), 'utf-8');
+
+  test('/welcome endpoint exists in server.ts', () => {
+    expect(serverSrc).toContain("url.pathname === '/welcome'");
+  });
+
+  test('/welcome serves HTML content type', () => {
+    const welcomeSection = serverSrc.slice(
+      serverSrc.indexOf("url.pathname === '/welcome'"),
+      serverSrc.indexOf("url.pathname === '/health'"),
+    );
+    expect(welcomeSection).toContain("'Content-Type': 'text/html");
+  });
+
+  test('/welcome redirects to about:blank if no welcome file found', () => {
+    const welcomeSection = serverSrc.slice(
+      serverSrc.indexOf("url.pathname === '/welcome'"),
+      serverSrc.indexOf("url.pathname === '/health'"),
+    );
+    expect(welcomeSection).toContain('302');
+    expect(welcomeSection).toContain('about:blank');
+  });
+});
+
+describe('headed launch navigates to welcome page', () => {
+  const serverSrc = fs.readFileSync(path.join(ROOT, 'src', 'server.ts'), 'utf-8');
+
+  test('server navigates to /welcome after startup in headed mode', () => {
+    // Navigation must happen AFTER Bun.serve() starts (not during launchHeaded)
+    // because the HTTP server needs to be listening before the browser requests /welcome
+    const afterServe = serverSrc.slice(serverSrc.indexOf('Bun.serve('));
+    expect(afterServe).toContain('/welcome');
+    expect(afterServe).toContain("getConnectionMode() === 'headed'");
+  });
+
+  test('welcome navigation does NOT happen in browser-manager (too early)', () => {
+    const bmSrc = fs.readFileSync(path.join(ROOT, 'src', 'browser-manager.ts'), 'utf-8');
+    // browser-manager.ts should NOT navigate to /welcome because the server
+    // isn't listening yet when launchHeaded() runs
+    const launchHeadedSection = bmSrc.slice(
+      bmSrc.indexOf('async launchHeaded('),
+      bmSrc.indexOf('// Browser disconnect handler'),
+    );
+    expect(launchHeadedSection).not.toContain('/welcome');
+  });
+});
+
+describe('sidebar auto-open (background.js)', () => {
+  const bgSrc = fs.readFileSync(path.join(ROOT, '..', 'extension', 'background.js'), 'utf-8');
+
+  test('autoOpenSidePanel function exists with retry logic', () => {
+    expect(bgSrc).toContain('async function autoOpenSidePanel');
+    expect(bgSrc).toContain('attempt < 5');
+  });
+
+  test('auto-open fires on install AND on every service worker startup', () => {
+    // onInstalled fires on first install / extension update
+    expect(bgSrc).toContain('chrome.runtime.onInstalled.addListener');
+    expect(bgSrc).toContain('autoOpenSidePanel()');
+    // Top-level call fires on every service worker startup
+    const topLevelCalls = bgSrc.match(/^autoOpenSidePanel\(\)/gm);
+    expect(topLevelCalls).not.toBeNull();
+    expect(topLevelCalls!.length).toBeGreaterThanOrEqual(1);
+  });
+
+  test('retry uses backoff delays (not fixed interval)', () => {
+    expect(bgSrc).toContain('500');
+    expect(bgSrc).toContain('1000');
+    expect(bgSrc).toContain('2000');
+    expect(bgSrc).toContain('3000');
+    expect(bgSrc).toContain('5000');
+  });
+
+  test('auto-open uses chrome.sidePanel.open with windowId', () => {
+    expect(bgSrc).toContain('chrome.sidePanel.open');
+    expect(bgSrc).toContain('windowId');
+  });
+
+  test('auto-open logs success and failure for debugging', () => {
+    expect(bgSrc).toContain('Side panel opened on attempt');
+    expect(bgSrc).toContain('Side panel auto-open failed');
+  });
+});
+
+describe('extension dispatches gstack-extension-ready event', () => {
+  const contentSrc = fs.readFileSync(path.join(ROOT, '..', 'extension', 'content.js'), 'utf-8');
+
+  test('content.js dispatches gstack-extension-ready CustomEvent', () => {
+    expect(contentSrc).toContain("new CustomEvent('gstack-extension-ready')");
+  });
+});

extension/background.js

@@ -406,29 +406,35 @@ if (chrome.sidePanel && chrome.sidePanel.setPanelBehavior) {
   chrome.sidePanel.setPanelBehavior({ openPanelOnActionClick: true }).catch(() => {});
 }
 
-// Auto-open side panel on install/update AND every service worker startup.
-// onInstalled fires on first install / extension update.
-// The startup block below fires every time the browser launches (persistent context reuse).
-chrome.runtime.onInstalled.addListener(async () => {
-  setTimeout(async () => {
+// Auto-open side panel with retry. chrome.sidePanel.open() can fail silently
+// if the window/tab isn't fully ready yet. Retry up to 5 times with backoff.
+async function autoOpenSidePanel() {
+  if (!chrome.sidePanel?.open) return;
+  for (let attempt = 0; attempt < 5; attempt++) {
     try {
-      const [win] = await chrome.windows.getAll({ windowTypes: ['normal'] });
-      if (win && chrome.sidePanel?.open) {
-        await chrome.sidePanel.open({ windowId: win.id });
+      const wins = await chrome.windows.getAll({ windowTypes: ['normal'] });
+      if (wins.length > 0) {
+        await chrome.sidePanel.open({ windowId: wins[0].id });
+        console.log(`[gstack] Side panel opened on attempt ${attempt + 1}`);
+        return; // success
       }
-    } catch {}
-  }, 1000);
+    } catch (e) {
+      // May throw if window isn't ready or user gesture required
+      console.log(`[gstack] Side panel open attempt ${attempt + 1} failed:`, e.message);
+    }
+    // Backoff: 500ms, 1000ms, 2000ms, 3000ms, 5000ms
+    await new Promise(r => setTimeout(r, [500, 1000, 2000, 3000, 5000][attempt]));
+  }
+  console.log('[gstack] Side panel auto-open failed after 5 attempts');
+}
+
+// Fire on install/update
+chrome.runtime.onInstalled.addListener(() => {
+  autoOpenSidePanel();
 });
 
-// Also auto-open on every browser launch (not just install)
-setTimeout(async () => {
-  try {
-    const [win] = await chrome.windows.getAll({ windowTypes: ['normal'] });
-    if (win && chrome.sidePanel?.open) {
-      await chrome.sidePanel.open({ windowId: win.id });
-    }
-  } catch {}
-}, 1500);
+// Fire on every service worker startup (covers persistent context reuse)
+autoOpenSidePanel();
 
 // ─── Tab Switch Detection ────────────────────────────────────────
 // Notify sidepanel instantly when the user switches tabs in the browser.