Barret is able to reproduce this quite often, and has done so a few days in a row, so cc'ing. Also cc'ing mstange who helped poke at Barret's machine while in the broken state.
Hey sotaro, mstange suggested you as someone who might have a sense of what's going wrong here. Specifically, the screenshot in comment 5 got mstange's attention: the video frames are scaled to the right size, but their position is offset as if the window (before resizing) was originally at (0,0), and then the rectangle where the video becomes visible is the intersection between the actual window and the window had it been at (0,0) before resizing.
We also noticed that this bug also occurs for the chrome of the window that Gecko draws in the parent process (we confirmed this by adding a border around the remote <browser> which renders in the parent process).
Is it possible that the compositor is confused about where the window is that it's supposed to be rendering into? Do you have any ideas of what we could do to debug this?