This is rather odd. We were getting reports of students unable to click their mouse pointer in certain areas of the screen.
Further investigation reveals that this is an empty, invisible window named "cinnamon-screensaver" using xwininfo. Not quite sure how to approach this, but we would prefer a surgical fix to a brute force, nuke-it-from-orbit approach.
I somewhat suspect this may have something to do with saved session data, and something autostarting, but so far have not found the solution.
I CAN tell you, that removing cinnamon-screensaver is NOT the fix. The cinnamon GUI is NOT happy without it.