Windowed 1280x720: doesn't work!

Well, I’m one of the unlucky guys that doesn’t have a fullHD monitor, my max res is 1280x1024, and if I want to film in HD I have to set Garry’s Mod to 1280x720 windowed.

I have an Ati Radeon HD4850 video card.

This is what happens:

I didn’t find any help on this. It’s odd because I tried this in a different game and it works.
Any suggestions :slight_smile: ? [EDIT] So nobody knows ? :\