Quote Originally Posted by funicorn View Post
So this means nvidia driver cannot get the actual size of the display. That's funny.
Nothing to do with ^
Option "ConstrainCursor" "boolean"

When this option is enabled, the mouse cursor will be constrained to the region of the desktop that is visible within the union of all displays' panning domains in the current MetaMode. When it is disabled, it may be possible to move the cursor to regions of the X screen that are not visible on any display.

Note that if this would make display's panning domain is inaccessible (in other words, if the union of all panning domains is disjoint), then the cursor will not be constrained, so that it is still possible to move the cursor to each display.

Default: on, if the X server supports it. The cursor will be constrained to the panning domain of each monitor, when possible.
The changelog does mention a bug fix for panning that possibly has created this which will no doubt be fixed

Fixed a bug that prevented panning from working correctly
after a modeswitch on some X servers with support for
cursor constraining.