Comment 1 for bug 45685

It looks like this happens when the screensaver activates when on a typing break. It still doesn't seem to happen every time.

I found that if I kill gnome-screensaver, everything returns to normal.

It has been suggested to me that this might be an interaction between OpenGL screensavers and Nvidia's X server. I've turned my screensaver to be a plain blank screen, and will report back with my results.