The "context menus" which normally show up on right-click, as well as other pull-downs (for example evince's resize screen or any program's at-the-top menu pulldowns) are invisible.
This is Ubuntu 12.04.
I know the computer still thinks it's drawing them because of the way left-click responds. But I can't see its contents.
What program draws these?
Found the answer here: How can I restart the menu bar in Unity?
Restarting
unitysolved the problem. This can be accomplished withoutkillall unity; I just didunityand that worked.