fix(desktop): stop the lane controller clobbering a mode another owner set

A regression I introduced last round. `DesktopTitleBarController` seeded `inset`
unconditionally on mount, before its own `getState()` resolved. That was harmless while
only `AuthShell` mounted it — but giving the file viewer a lane put a controller inside
workspace for the first time, exactly where this PR's own comments say `WorkspaceChrome`
owns the mode. Opening a file during native fullscreen therefore snapped the traffic-light
lane back on and jumped the content, and left it wrong permanently if `getState()`
rejected, since the rejection is swallowed.

It now seeds only when no owner has established a mode. The pre-paint script sets the
marker before first paint and `WorkspaceChrome` maintains it, so the unconditional write
was never the thing making the lane correct — it was only ever able to make it wrong.

Adds a controller test covering the case directly: mount during `fullscreen` with a
`getState` that never settles, and the mode survives. Verified it fails against the
previous behaviour.

Author: waleedlatif1

apps/sim/app/_shell/desktop-title-bar-controller.test.tsx

@@ -0,0 +1,86 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { act } from 'react'
+import { createRoot, type Root } from 'react-dom/client'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+const { mockGetDesktopBridge } = vi.hoisted(() => ({ mockGetDesktopBridge: vi.fn() }))
+
+vi.mock('@/lib/desktop', () => ({ getDesktopBridge: mockGetDesktopBridge }))
+
+import {
+  DESKTOP_TITLE_BAR_ATTRIBUTE,
+  DesktopTitleBarController,
+} from '@/app/_shell/desktop-title-bar'
+
+let container: HTMLDivElement
+let root: Root
+
+/**
+ * A bridge whose `getState` never settles, which is the window this guards: the gap
+ * between mount and the async state arriving is exactly when the old code clobbered
+ * whatever mode another owner had already established.
+ */
+function pendingBridge() {
+  return {
+    windowState: {
+      onStateChange: vi.fn(() => vi.fn()),
+      getState: vi.fn(() => new Promise<never>(() => {})),
+    },
+  }
+}
+
+function mount() {
+  ;(globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true
+  container = document.createElement('div')
+  document.body.appendChild(container)
+  root = createRoot(container)
+  act(() => root.render(<DesktopTitleBarController />))
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  document.documentElement.removeAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE)
+  Object.defineProperty(navigator, 'userAgent', {
+    value: 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7)',
+    configurable: true,
+  })
+})
+
+afterEach(() => {
+  act(() => root.unmount())
+  container.remove()
+  document.documentElement.removeAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE)
+})
+
+describe('DesktopTitleBarController', () => {
+  it('leaves an established mode alone while its own state is still pending', () => {
+    mockGetDesktopBridge.mockReturnValue(pendingBridge())
+    // Native fullscreen, set by WorkspaceChrome. A lane-aware overlay mounting here used
+    // to seed `inset` first, snapping the traffic-light lane back on and jumping the
+    // content until the async state corrected it — or permanently, if `getState` rejected.
+    document.documentElement.setAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE, 'fullscreen')
+
+    mount()
+
+    expect(document.documentElement.getAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE)).toBe('fullscreen')
+  })
+
+  it('seeds inset when no owner has set a mode yet', () => {
+    mockGetDesktopBridge.mockReturnValue(pendingBridge())
+
+    mount()
+
+    expect(document.documentElement.getAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE)).toBe('inset')
+  })
+
+  it('clears the marker off the desktop shell', () => {
+    mockGetDesktopBridge.mockReturnValue(null)
+    document.documentElement.setAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE, 'inset')
+
+    mount()
+
+    expect(document.documentElement.hasAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE)).toBe(false)
+  })
+})

apps/sim/app/_shell/desktop-title-bar.tsx

@@ -5,6 +5,9 @@ import { getDesktopBridge } from '@/lib/desktop'
 
 export type DesktopTitleBarMode = 'fullscreen' | 'inset' | null
 
+/** The document marker every lane owner reads and writes. */
+export const DESKTOP_TITLE_BAR_ATTRIBUTE = 'data-sim-desktop-title-bar'
+
 /**
  * Whether this surface reserves the macOS traffic-light lane itself.
  *
@@ -25,10 +28,10 @@ export function applyDesktopTitleBarMode(
   mode: DesktopTitleBarMode
 ): void {
   if (mode === null) {
-    root.removeAttribute('data-sim-desktop-title-bar')
+    root.removeAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE)
     return
   }
-  root.setAttribute('data-sim-desktop-title-bar', mode)
+  root.setAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE, mode)
 }
 
 /**
@@ -64,7 +67,17 @@ export function DesktopTitleBarController() {
     }
 
     const windowState = bridge?.windowState
-    applyDesktopTitleBarMode(root, 'inset')
+    /**
+     * Seed `inset` only when nobody has established a mode yet.
+     *
+     * The pre-paint script sets this before first paint and `WorkspaceChrome` maintains it
+     * for workspace routes, so writing unconditionally clobbered whatever they had:
+     * mounting a lane-aware overlay during native fullscreen forced the traffic-light lane
+     * back on and jumped the content, and left it wrong entirely if `getState()` rejected.
+     */
+    if (!root.hasAttribute(DESKTOP_TITLE_BAR_ATTRIBUTE)) {
+      applyDesktopTitleBarMode(root, 'inset')
+    }
     if (!windowState) return
 
     let disposed = false