Recently i have upgraded from Ubuntu 16 to Ubuntu 18. Usually i use Mate desktop and the upgrade was made in Mate session via command line. The problem is, that when i log into Ubuntu desktop it is as if scaled to 200% i can see only one quarter of the desktop and i can reach other corners with mouse. I have checked the scale option in settings: it is 100%, so the problem is not here. When i log back to Mate, it is all right. I have rebooted several times and ran "sudo dpkg --configure -a" (if it matters).
SnapOverflow
It was zoom (in Universal access) enabled.