The FFT/scope surfaces are fed by the local engine's writer thread, which
is stopped while a renderer plays, so every visualizer animation froze
the moment a cast started. cast_viz decodes the same bytes the renderer
is served (progressive download buffer, cached track or local file)
silently on its own thread and pushes them into the VisualizerTap at
real-time pace, re-anchored to the renderer's reported position on each
poll (re-seek past 0.75 s of drift, paused with the renderer). Proxied
server-streamed tracks have no local bytes and stay dark.
cc
vicrodh
a8796ab91e