How do I keep full screen when losing focus on dual monitor?

[Ralph Wiggum]

I have a two monitors. When I open a youtube video, there's an option to watch the video in full screen mode. What I'd like to do is watch it on one monitor in full screen mode, while doing my work on the 2nd monitor.

However, as soon as I click elsewhere on the screen, the video returns from full screen to regular mode. How do I correct this?

[kerowo]

Are you using Multimon?