How to make YouTube fullscreen actually fullscreen?

Okay, this is one of those embarassing occasions when you discover what's going on just after posting a question...

It is because of my dual monitor setup. YouTube is seeing the size of both monitors together, but then shrinks it to fit on one for some reason.

It doesn't happen on videos that are available in HD (in fact if the video is available in HD even if you watch it at 480p or lower resolutions then it works, which is a bit strange).

It seems this issue has been around for several years and the easiest fix is to enable HTML5 mode in YouTube.

I think the best solution is using the MaximizeFlash plugin for Chrome. It detects any flash in the page and you can make them fullscreen. To get rid of toolbar up just hit f11. You don't need to install any other apps.

For Firefox there is a similar plugin.