Every seek on Linux stalled the UI thread: the playback-restart handler read `time-pos` with a synchronous mpv_get_property from the GLib main context, which parks that thread on mpv's playloop for as long as the core takes — and the core's render path can itself be waiting on the main thread to post the next render job.
The position is now requested through the existing asynchronous property registry when the restart is dequeued, with the source identity captured at that moment, and the unchanged playback-restart envelope is sent when the reply lands. A source boundary — start-file or end-file — delivers any restart still waiting as a position-less envelope for its captured source and retires the reply, so no restart is ever reported after its source's boundary or attributed to the next source. No new event type or thread.
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