preview_window_capacity used the *configured* worker count, so a
window opened while workers were still handshaking (or after a crash)
could claim every slot of the smaller live pool — the synchronous
render ticket then never gets a free slot, and since the slot-releasing
cleanup runs on the same UI thread that is blocked in TicketArena::wait,
playback deadlocks permanently. Intermittent on Linux CI (the
playback_display_tracks_the_playhead hang, caught by the new test
watchdog): depends on how many workers had handshaken when playback
started. Count only Alive workers (fall back to the configured count
while none are alive, keeping the existing unit test semantics).
Mike-Solar
40c8043a76