Background Video Displays Wrong Size

I am using BB with Tesseract theme. Whatever I do, I cannot get the background video to size properly - it displays larger than the window (even at full screen) but snaps back to correct size as soon as the window size is adjusted.

I have tried everything I can find including resizing the video to 1200x720.

It’s 5 seconds long, MP4 and only 1.3 MB

Help please!