fix(browser): guard auth-host UA switch and cover it in tests

did-start-navigation now reads the guest UA on every main-frame nav; the
load-failure replay guest mocks had no WebContents UA surface, so the handler
threw. Only touch the session UA when actually leaving an auth host (a Firefox
override is in place), give the affected mocks a faithful UA surface, and add
direct coverage for the auth-host switch, off-host restore, and native opt-out.

src/main/browser/browser-manager.test.ts

@@ -55,9 +55,17 @@ vi.mock('./popup-origin-bar-window', () => ({
 }))
 
 import { browserManager } from './browser-manager'
+import { googleAuthUserAgent } from './browser-google-auth-ua'
 
 describe('browserManager', () => {
   const rendererWebContentsId = 5001
+  // Base (non-Firefox) UA a guest reports off the Google auth hosts.
+  const guestBaseUserAgent = 'Mozilla/5.0 (Test) Chrome/140.0.0.0'
+  const guestUaMethods = () => ({
+    getUserAgent: vi.fn(() => guestBaseUserAgent),
+    setUserAgent: vi.fn(),
+    session: { getUserAgent: vi.fn(() => guestBaseUserAgent) }
+  })
   type DownloadItemHandlerState = 'progressing' | 'interrupted' | 'completed' | 'cancelled'
   type DownloadItemHandler = (event: Electron.Event, state: DownloadItemHandlerState) => void
 
@@ -1610,7 +1618,8 @@ describe('browserManager', () => {
       on: guestOnMock,
       off: guestOffMock,
       openDevTools: guestOpenDevToolsMock,
-      getURL: vi.fn(() => 'http://localhost:3000/')
+      getURL: vi.fn(() => 'http://localhost:3000/'),
+      ...guestUaMethods()
     }
 
     webContentsFromIdMock.mockImplementation((id: number) => {
@@ -1686,7 +1695,8 @@ describe('browserManager', () => {
       on: guestOnMock,
       off: guestOffMock,
       openDevTools: guestOpenDevToolsMock,
-      getURL: vi.fn(() => 'https://example.com/')
+      getURL: vi.fn(() => 'https://example.com/'),
+      ...guestUaMethods()
     }
     webContentsFromIdMock.mockImplementation((id: number) =>
       id === guest.id
@@ -1738,7 +1748,8 @@ describe('browserManager', () => {
       off: guestOffMock,
       openDevTools: guestOpenDevToolsMock,
       send: vi.fn(),
-      getURL: vi.fn(() => 'chrome-error://chromewebdata/')
+      getURL: vi.fn(() => 'chrome-error://chromewebdata/'),
+      ...guestUaMethods()
     }
     webContentsFromIdMock.mockReturnValue(guest)
 
@@ -1780,7 +1791,8 @@ describe('browserManager', () => {
       off: guestOffMock,
       openDevTools: guestOpenDevToolsMock,
       send: vi.fn(),
-      getURL: vi.fn(() => 'chrome-error://chromewebdata/')
+      getURL: vi.fn(() => 'chrome-error://chromewebdata/'),
+      ...guestUaMethods()
     }
     webContentsFromIdMock.mockReturnValue(guest)
 
@@ -1819,6 +1831,85 @@ describe('browserManager', () => {
     expect(browserManager.getBrowserPageLoadError('browser-retry-abort-page')).toBeNull()
   })
 
+  it('presents the Firefox UA on Google auth hosts and restores the base UA off them', () => {
+    let currentUa = guestBaseUserAgent
+    const setUserAgent = vi.fn((ua: string) => {
+      currentUa = ua
+    })
+    const guest = {
+      id: 408,
+      isDestroyed: vi.fn(() => false),
+      getType: vi.fn(() => 'webview'),
+      setBackgroundThrottling: guestSetBackgroundThrottlingMock,
+      setWindowOpenHandler: guestSetWindowOpenHandlerMock,
+      on: guestOnMock,
+      off: guestOffMock,
+      openDevTools: guestOpenDevToolsMock,
+      getURL: vi.fn(() => 'https://accounts.google.com/'),
+      getUserAgent: vi.fn(() => currentUa),
+      setUserAgent,
+      session: { getUserAgent: vi.fn(() => guestBaseUserAgent) }
+    }
+    webContentsFromIdMock.mockReturnValue(guest)
+
+    browserManager.attachGuestPolicies(guest as never)
+    browserManager.registerGuest({
+      browserPageId: 'browser-auth-ua',
+      webContentsId: guest.id,
+      rendererWebContentsId
+    })
+    const didStartNavigation = guestOnMock.mock.calls.find(
+      ([event]) => event === 'did-start-navigation'
+    )?.[1] as (event: unknown, url: string, isInPlace: boolean, isMainFrame: boolean) => void
+    setUserAgent.mockClear()
+
+    didStartNavigation(null, 'https://accounts.google.com/v3/signin/identifier', false, true)
+    expect(setUserAgent).toHaveBeenLastCalledWith(googleAuthUserAgent())
+
+    // Off the auth host, the guest's base identity is restored.
+    didStartNavigation(null, 'https://myaccount.google.com/', false, true)
+    expect(setUserAgent).toHaveBeenLastCalledWith(guestBaseUserAgent)
+
+    // A navigation that doesn't change the required UA must not thrash setUserAgent.
+    setUserAgent.mockClear()
+    didStartNavigation(null, 'https://example.com/', false, true)
+    expect(setUserAgent).not.toHaveBeenCalled()
+  })
+
+  it('leaves the UA untouched on Google auth hosts for native-UA profiles', () => {
+    const setUserAgent = vi.fn()
+    const guest = {
+      id: 409,
+      isDestroyed: vi.fn(() => false),
+      getType: vi.fn(() => 'webview'),
+      setBackgroundThrottling: guestSetBackgroundThrottlingMock,
+      setWindowOpenHandler: guestSetWindowOpenHandlerMock,
+      on: guestOnMock,
+      off: guestOffMock,
+      openDevTools: guestOpenDevToolsMock,
+      getURL: vi.fn(() => 'https://accounts.google.com/'),
+      getUserAgent: vi.fn(() => guestBaseUserAgent),
+      setUserAgent,
+      session: { getUserAgent: vi.fn(() => guestBaseUserAgent) }
+    }
+    webContentsFromIdMock.mockReturnValue(guest)
+
+    browserManager.attachGuestPolicies(guest as never)
+    browserManager.registerGuest({
+      browserPageId: 'browser-native-ua',
+      webContentsId: guest.id,
+      rendererWebContentsId,
+      userAgentMode: 'native'
+    })
+    const didStartNavigation = guestOnMock.mock.calls.find(
+      ([event]) => event === 'did-start-navigation'
+    )?.[1] as (event: unknown, url: string, isInPlace: boolean, isMainFrame: boolean) => void
+    setUserAgent.mockClear()
+
+    didStartNavigation(null, 'https://accounts.google.com/v3/signin/identifier', false, true)
+    expect(setUserAgent).not.toHaveBeenCalled()
+  })
+
   it('queues permission denials and download requests until the guest registers', () => {
     const rendererSendMock = vi.fn()
     const guest = {

src/main/browser/browser-manager.ts

@@ -926,9 +926,16 @@ export class BrowserManager {
     if (mode === 'native') {
       return
     }
-    const desiredUa = isGoogleAuthUrl(url) ? googleAuthUserAgent() : guest.session.getUserAgent()
-    if (guest.getUserAgent() !== desiredUa) {
-      guest.setUserAgent(desiredUa)
+    const firefoxUa = googleAuthUserAgent()
+    const currentUa = guest.getUserAgent()
+    if (isGoogleAuthUrl(url)) {
+      if (currentUa !== firefoxUa) {
+        guest.setUserAgent(firefoxUa)
+      }
+    } else if (currentUa === firefoxUa) {
+      // Only restore when the auth-host override is actually in place, so normal
+      // navigation never touches the session UA.
+      guest.setUserAgent(guest.session.getUserAgent())
     }
   }