High Sierra 10.13.4 Mission Control Hot Corners Broken
Apple Support was willing to help me troubleshoot, even though I don't have a current support contract.  We tried several things, and eventually determined that a desktop/window manager add-on called Magnet was messing up the UI events.  (I had tried just turning off the add-on, but that didn't work.)  It was some other troubleshooting step, including:


Reset NVRAM (shutdown, power up with Opt+Cmd+P+R)
Safe-mode Boot (Restart, hold Shift during boot)
Create new admin user, log in (this helps distinguish a system problem from a particular user's preferences problem)
Disable startup items


Resetting NVRAM didn't help, but safe mode worked.  (However, safe mode turns off all graphics acceleration, so it's really slow on a Retina machine.)

The new user didn't show the problem, so it was clearly something in my preferences / profile.

Finally, turning off Magnet auto-start and rebooting cleared up the problem in my user account.  I was subsequently able to turn Magnet on again, and it's working OK now.  So I suspect it was a stray preference that conflicted with an OSX upgrade, which got cleared out by the rebooting and other troubleshooting steps.

I'm extremely grateful to Apple Support for the help!