X11 has been relatively sane on my Arch install until recently. It behaves normally in most programs, but as of maybe a week ago, if anything tries to run fullscreen it will only update when my mouse moves. For example, screensavers and youtube videos (Firefox) only work when my mouse is moving, somewhat defeating the purpose of both.
This issue occurs both under GNOME and in a naked X.org session running without a window manager.
Unforunately I've been unable to find anyone else with this or a similar problem through Google, but I very well may have been using poor search terms.