Yesterday suddenly my output (fullscreen on 2nd monitor) moved down so I could see my desktop background image... I re-enabled the display and it was fine again. - Not a big problem.
I can reproduce this, by moving the mouse to the right (out of the GUI, where the 2nd screen is), when I move the mouse up to the top of the 2nd screen, the mouse cursor appears and I can drag the output "window" (which is actually fullscreen).
As I said, not a big problem, as long as it does not happen accidentally.
Maybe there is a way to "freeze" the position of the output fullscreen