Comment 6 for bug 1043575

I can confirm this happens with Virtualbox guest drivers on (guest) Xubuntu 12.10, running on a host with Virtualbox 4.1.2, Xubuntu 11.10 with Nvidia graphics hardware (on which the Xfce glitch mentioned does not present... :-))

Deleting everything in $HOME/.cache *does not* fix this specific problem.

Debian testing (Wheezy)'s version, 2:1.12.4-1, is unaffected by this on Intel graphics hardware *and* on a VirtualBox instance with identical guest graphics and host machine to the affected Xubuntu release. In the latter case, the fact that it works is almost certainly wholly due to Debian's revert of the upstream change introducing this regression as a fix for debbugs #688908.

In short:

 Unaffected: Xubuntu 11.10 + xorg 2:1.10.4-1ubuntu4.3 + xfwm4 4.8.2-0ubuntu1 + nvidia-current 280.13-0ubuntu6.2
 Affected: Xubuntu 12.10 + xorg 2:1.13.0-0ubuntu6 + xfwm4 4.10.0-2ubuntu1 + virtualbox-guest-x11 4.1.18-dfsg-1ubuntu1
 Unaffected: Debian wheezy (testing current) + xorg 2:1.12.4-1 + xfwm4 4.8.3-2 + xserver-xorg-video-intel (2:2.19.0-6)
 Unaffected: Debian wheezy (testing current) + xorg 2:1.12.4-1 + xfwm4 4.8.3-2 + virtualbox-guest-x11 4.1.18-dfsg-1.1

Hopefully the Debian solution will be forwardported into Xubuntu at some point, or upstream will address it and we'll get the fix that way.