restart hot corners

Have you tried restarting SystemUIServer? (killall SystemUIServer) When something like his is happening and restarting the Dock doesn't help, SystemUIServer is the next one to try.

It happened again, and I tried simply closing the MacBook, waiting a couple of seconds, and reopening. Maybe it was a fluke, but it worked. I'll wait and see if it fixes the problem a second time to make sure that it really did something.

This answer is less than ideal, e.g., if I'm in the middle of a big download, so I'd like to wait and see if this answer can help towards a better solution. What does suspend trigger that I could do without suspending?