The main-process sample showed the UI thread spending 100% of its time
in TicketArena::wait from the painted frame's synchronous render: every
cache-missed playhead frame sync-rendered inline, and the seek-priority
ticket then stole worker capacity from the pre-render window while the
blocked tick loop could not feed it — a self-reinforcing loop that made
playback unusably choppy.
On a playback miss the viewer now shows the last displayed frame while
the pre-render window warms up/catches up (paused monitors and the very
first frame keep the synchronous path). Adds a gpui test driving real
playback that requires the window to supply playhead frames, and the
real-footage bench_playback example used for the measurements
(152 fps aggregate on 1080p H.264 at 480p preview, decode-bound).
Mike-Solar
9e9c6d9863