I’m using the new Edge with an extension YouTube Popout Player, that pops the video out to a new window. (I then use a shortcut in AutoHotKey to keep the popped out window on top of everything.) Anyway, when I press the play/pause media key on my keyboard, it starts/stops the popped out YouTube video. I popped a window out using the same extension on Chrome, and it doesn’t start/stop the videos. I’m assuming this was something added to Edge, which is a nice touch.