Streaming videos skip on_image_done (the _load coroutine returns
early), so the thumbnail's _cached_path was never set. Drag-to-copy
failed until the user navigated away and back (which went through
the cached path and hit on_image_done).
Now on_video_stream sets _cached_path to the expected cache location
immediately. Once the stream-record promotes the .part file on EOF,
drag-to-copy works without needing to change posts first.
pax
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