It seems that the biggest resolution in the mode list of the primary screen in xorg.conf is set to be the desktop size of the primary screen, but the first item in the mode list is set to be the primary display resolution. That is why they can differ. I can get rid of the problem by putting the biggest (and preferred) resolution first in the list.
-- Desktop size bigger than monitor resolution on the primary screen when using Xinerama https://launchpad.net/bugs/61727 -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
