Runner creates a Screencaster before sessions are acquired, then
wires its start/stop into DashboardServer.onStartStream / onStopStream
after sessions exist (the handlers close over a sessionsByKey map).
Clicking a player tile in the dashboard starts a CDP screencast on
that session's page, forwards JPEG frames as WS "frame" messages.
Closing the modal (or disconnecting the WS) stops all screencasts.
Verified end-to-end: programmatically connected WS, sent start_stream,
received 5 frames (13.7KB each), sent stop_stream, screencast_stopped
log line fired, run completed cleanly.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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