X11/GLX - Fullscreen mode?

Question

I am trying to create a Linux application - a screensaver, in this case - and it is proving remarkably difficult to find information on the simple task of making a wind

Answer 1

The piece of information you lack is, that the screensavers are not responsible for going fullscreen. The screensaver daemon will manage the screensaver window, put it into the dedicated screensaver window layer and make it full screen.

So for writing a screensaver you're in the clear. If you were about writing a fullscreen game, you's have to set the Override Redirect attribute to prevent the window from getting managed by the WM and make it cover the whole screen.