Comment 19 for bug 603482

Fred, you are right of course, but the underlying issue is well known. The X Window specification lets an application "grab" the mouse pointer or the keyboard, preventing any other application from using them until they are released again. Like many things in the X Window System, this only works if the application behaves itself and doesn't have a bug in the wrong place. There would probably be ways to work around this - X has a "give me back my mouse" key combination which you can activate - but fundamentally this can only be changed by replacing X with something else (Wayland?) And hoping that the replacement doesn't have any equally silly problems that no one thought of in time!

Take a look at http://www.x.org/wiki/Development/X12 if you are interested in other shortcomings of X11.