Linux Mouse don't reach lower section of the screen

Welcome to the XBMC forums.

Try what I did in post #6 above by switching to Unity. Toggle back and forth between fullscreen and windowed and see if the problem goes away. If necessary, resize the XBMC window and then toggle to fullscreen and see if problem persists. Once the problem is gone in Unity, switch back to Gnome Classic and see if the problem is gone there as well.

Dear Artrafael,

Thank yuo for your help! I did switch back my Ubuntu 12.04 from Gnome-Classic to original Unity and my problem is solved! The mouse cursor works perfectly in XBMC full screen mode. But unfortunatly I d'ont like Unity. Then I did install Cairo-Dock. I did try to use Cairo-Dock as a shell and XBMC still works fine in full screen mode.
Thanx, DFeri

Glad it worked out for you! I'm no fan of Unity either, but that's another story.