Hello,
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.
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Thanks a ton for this hint, that issue has been bugging me for ages. 
I experienced that adding the "export GDK_CORE_DEVICE_EVENTS=1" caused the plank dock to crash when moving icons. (Using Cupertino panel layout)
There are at least two references for this undesired behavior:
A solution for this is to fix xorg-server with a patch, which is referenced here:
I tried compiling xorg-server with this patch and it solves the issue. With that, exporting GDK_CORE_DEVICE_EVENTS was not necessary.
If someone is interested, I made a sh script which uses the source file of the installed xorg-server, patches it and creates a new deb file, which it installs when finished. This is done in a chroot environment preventing cluttering of the file system.
The script can be found here:
Although the commit has been backported to the xorg-server 21.1 branch, I have not found it in the branch which Ubuntu provides (Which is at the moment also using 21.1).
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