I'm on Ubuntu 12.04, which I have had for a long time. On this computer I've installed both XFCE4 and Gnome 3, and I'm currently using Gnome 3 Fallback.
Recently I've been having a weird problem. When I close all open windows in a given workspace (be it with mouse, alt-f4 or other key-combinations), everything loses focus and the keyboard doesn't work anymore. I need to click anywhere on the desktop for these things to start working again.
What can I do to fix this problem?