Display setting quirck with secondary display above primary

[Lorenzo Sutton]

I have recently phisically moved my external (secondary) display from 
being on the right of my laptop (primary display) above it on a stand.

If in the display settings I put (by dragging it) the external display 
above the laptop one (primary) all windows on the primary display move 
to that external one. The only thing which remains as should are the two 
(different) XFCE panels. I noticed that if I put the external display 
_below_ the laptop one in the display manager windows remain on the main 
one as expected, with the caveat that I need to move the mouse in 
opposite directions of what is actually the physical world.

I noticed that this also happens if the external (seconday) display is 
positioned _to the left_ of the primary one. Everything works as 
expected if the external display is on the right of the primary one 
(which I'm keeping as it is the configuration which works).

Any ideas?

Lorenzo