From a boot, move the mouse up a long way (like to the top of the screen) then bring it down slowly.
When you see things change on your mangled screen the pointer is probably hovering over the first (maybe only) username. Now single click, type password and ENTER.
If you want to try lightdm it's easy from the console :
Code:
sudo apt install lightdm
I haven't tried this on my Mac yet so I can't tell you if it fixes the problem (assuming you, like me, have no problems with the desktop itself once you get into it). I'll try it myself over the weekend...
Once you have lightdm instead of gdm, the username and password box is already selected when you boot so there's no mouse clicking needed, just password and ENTER.
EDIT : Lightdm works perfectly on my Mac.
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