As video track of the remote stream is naturally a push model, but compositing is a pull model, there must be a pool somewhere (now it is the WebMediaPlayerMS).

Compositing pull happens on compositor thread.
We mainly use IO thread pushing the frames along the code path, but from MediaStreamVideoRendererSink to WebMediaPlayerMS, we use render main thread to do the push.

I think what you mean is that we can avoid using renderer main thread, but stick to using IO thread to push the frame into WebMediaPlayerMS.

Tagging with current canary milestone.

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This one is already fixed long time ago.