When clicking the "Expand player to window size" button (or when pressing the shortcut 'x') it just centers the video player on my screen with grayed out sides and black above and below the image. It looks like it's trying to format the video to be played on a phone that's being held vertically. It also cuts off have the video player options at the bottom of the screen.

To be clear, that is a screenshot of my entire screen (without showing my bookmark bar and URL bar). This started a handful of updates ago.