This is on a new machine, freshly wiped and updated. It used to be a problem on other MacBooks I've owned, but it hasn't been a problem lately until now. If you fullscreen a video in youtube in chrome, some videos use OS X's fullscreen mode instead of just going fullscreen like every other video on the entire internet, and most videos on youtube. The details of why this is a huge problem are beyond the scope of this thread. How do I prevent Chrome from initiating OS X's fullscreen mode? Even Safari doesn't use OS X's fullscreen mode for youtube videos.