How to force quit any app.
GNOME Documentation Project
gnome-doc-list@gnome.org
Force Unresponsive Program to Close
To force quit any application:
Press AltF2.
A command window pops up. Type xkill. Your mouse point turns into an 'x'.
Click on the application to terminate it.
After you press AltF2, you can press Esc if you change your mind about force quiting the application, or if the application starts responding.
Explain Force Quit. Does the Shell gray-out apps that have frozen too, like Compiz does?