MediaStream canvas.captureStream() for Canvas element provides frames too late
To observe, obtain bug 230613 testcase mediastream-canvas-to-video.html
When canvas draws, the frame should go to the video.
Instead it doesn't go anywhere.
When the canvas draws a new frame, previous frame goes to video.
Will be fixed by bug 231022
*** This bug has been marked as a duplicate of bug 231022 ***
Scratch that, probably too risky. Should fix it separately
The frame should be captured and provided to video element during canvas display buffer preparation. However, at that moment the tree is already laid out. Submitting a frame will cause needs-layout assertion.