Ever since Ubuntu MATE 18.04, there was this issue where if the mouse cursor was at the very top left (really, the edge) of the screen and you tried to click the "Menu" button, it sometimes wouldn't register. The same applies for the "Show Desktop" button on the lower left. This was unfortunate since I would sometimes blindly fling the mouse all the way to the lower left to minimize everything.
Well, I think I found a workaround for the issue.
I added "export GDK_CORE_DEVICE_EVENTS=1" (without the quotes) to the .profile in my home directory and rebooted.
It seems to have resolved the issue. I can now click the button 100% of the time, even if the cursor is at the very edge of the screen.
I'm not sure what side effects this will have, but I don't really care since this bug was so annoying. I'm just glad it works now.
Just figured I'd post since I'm sure others are annoyed by this.