Problem using fullscreen

Hello there again!

Once again, I have a strange problem.
I have a nice little application, which only draws a green triangle.
The problem is, that the triangle gets drawn when not using fullscreen, and
not drawn in fullscreen.

It think it’s weird, since I use the same code in a similar program, and
there it works…

The program which bugs itself can be found here:
The program using similar code can be found in the subdirectory cirkelosv.

I honestly don’t see the problem.