Mouse in window (or border box?) after upgrading to High Sierra

Since upgrading to High Sierra v10.13.1, the mouse is constrained by a window or perhaps it is a border box?

There is no effect on functionality. It slides with the mouse when the mouse reaches the edge of the box!
Is this some feature that has been inadvertantly turned on?

It is about three quarters of the screen size and cannot be changed in size.

It can be pushed to anywhere by the Mouse and will stay there until pushed by the mouse on another edge.

I have re-installed High Sierra v10.13.1 but is still there.

It is most annoying!