Solved the resizing issue, at least partly.
If you swap “auto” to “responsive” then resizing the Web Window also resizes the video. Otherwise, not.
One incompatibility: if you put scrollbars on “Auto” it causes an ugly white border around the bottom and right of the edges of the video. If you leave that off it just letterboxes (black borders) whenever the Web Window aspect ratio is not identical to the video’s.