there is no "redraw screen" or "repaint screen" command to be found in GNOME/sawmill ( didn't check KDE ). In the old days fvwm had this in the main pop-up menu. This is needed when some app messes up the display, like xteevee -root -static , a demo from xscreensaver. Maybe it is a bug in X server , but a "redraw screen" command is nevertheless needed , IMHO.
This just doesn't happen that often in GNOME (at least in my experience) and when it does, most of us usually run xrefresh from a terminal. If you find you do this alot, you can make a launcher on the panel with xrefresh as the command.