Ah, I spoke too soon. Removing top:0 works great until it gets down to mobile. Then the video sits on the bottom of the container with a big black bar at the top, even after reducing the headline size. I put top:0 back in and of course the vid goes to the top of the container and the black bar goes to the bottom (which is better but still not ideal).
I have a feeling the second solution in that stackoverflow link I sent might be a good answer for matching up screen to container, but I couldn’t get it to work properly with the content that overlays the button.