Desktop browser tabs stay on the empty page after one tab detaches during server setup #34251

Area: apps/server (preview: ServerBrowser, DesktopBrowserChannel)

Steps to reproduce

The window is a few hundred milliseconds wide, so it needs the desktop to detach a tab while the
server is setting it up. To make it deterministic, add a temporary
await new Promise((r) => setTimeout(r, 2500)) right after connectDesktop in
ServerBrowser.setUpTab, then in the desktop app:

1. Open a browser tab in a thread.
2. Within those 2.5 s, make the desktop detach the tab (for example, its webview moves to another
   app window, or the tab's webview is recreated).
3. Open another browser tab and enter an address or a search.

Expected behavior

The new tab shows the page it loaded.

Actual behavior

The webview loads the page, but the panel keeps showing the empty page over it (the spinner
shows, then nothing). Every later user-opened desktop tab does the same until the app restarts.
Agent-opened tabs keep working.

Cause

When the desktop detaches a tab, DesktopBrowserChannel's detached handler shuts that tab's
inbound queue, but the relay WebSocket stays open: the endpoint's handler forks the queue→socket
writer and then loops on reader.pull forever. Playwright never sees a disconnect, so the next
CDP call in setUpTab after the connect (newCDPSession, the PDF navigation setup, or
Page.enable) never answers. ensureTab's promise for that tab never settles. The
desktopChannel.attached and manager.events consumers await ensureTab inside
Stream.runForEach, so every later attach or opened event waits behind it: no later desktop tab
is set up, its snapshot stays Idle, and the client renders the empty state. Agent tabs are set
up through findTab, outside those consumers.

Impact

High for desktop browser users: search, typed addresses and recent sites stop working in every
new tab until restart, with no error.

Version or commit

Code path unchanged on main at 5bb5d53. Observed in a downstream desktop build based on T3 at
44ebb2f.

Environment

macOS 26, desktop app, two app windows open.

Logs

Inspecting the running server's heap: pendingTabs held one tab whose promise never settled while
inbound for that key was Done; every tab attached afterwards was in attachedTabs but in
neither tabs nor pendingTabs, with snapshots Idle/Loading while the desktop webview showed
the loaded page.

Workaround

Restart the app.