`Start-UiServer` printed the -SelfTestUi URL (and opened the browser window)
as soon as the TcpListener was bound, but the runspace that answers /progress
starts asynchronously — BeginInvoke returns before the pipeline is open and
the script block is JIT'd, which is seconds on a loaded runner. The kernel
accepted connections into the backlog during that gap and nobody answered
them. The self-test hit it three times (#90371 and two follow-ups each
widened a timeout instead of removing the race) and it just failed an
unrelated hermes_state.py PR (run 33591547099, two 5s stale-backlog
timeouts = red).
- windows.ps1: readiness handshake after BeginInvoke — one /progress
round-trip must succeed (≤15s) before the server is returned; on failure
tear the listener down and continue without UI. The URL now means
"serving", not "bound". Also fixes the browser opening to a page that never
loads on a slow machine.
- test: 1s per-attempt probe timeout so a single dead backlog socket cannot
consume half the readiness budget.
- CI: new `desktop_updater` classifier lane. tests/test_desktop_update_windows_*.py
spawn the real PowerShell script; the Windows-only job now runs them only
when scripts/desktop-update/**, the Electron updater launcher, conftest,
pyproject, or those tests change (push/dispatch fail open). A PR that
never touched that surface cannot be failed by its process timing.
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