CAVEAT: XScreenSaver and other screensavers have known problems with VLC and
xfce4-power-manager. If you're using XFCE or VLC you may want to check out those first.
My problem is that with modern laser based mice, a slight vibration of desktop, causes mouse to send signal as "moved". So I would like to stop the XScreenSaver only with keyboard -- I could have XScreenSaver running and move the mouse with no effect on it (clicking could be ignored or not, I don't care).