Looks like this is the "fix" for the issue where the pop-out player did not keep aspect ratio (DNA-65862). Relevant XKCD: https://xkcd.com/1172/
Along with Salithtaydan, I liked the "broken" functionality better, being able to resize the player to fit the maximum amount of space I wanted it to take up. Now, when my YouTube playlist changes songs, and the video now is at a different resolution, the player changes and covers up part of the screen i did not want it to