Thanks! I see what you mean but now I see I have a different problem! My monitors are arranged with Number 2 on the left hand side and Number 1 on the right. The desktop preview seems to assume I have 1 on the left and 2 on the right. The rest of Dexpot is fine with my arrangement. This is what I see:
If I drag a window on the screen from monitor 1 to monitor 2 (right to left) then the sketch disappears off the preview window to the left hand side. If I drag a window on the preview from left to right, into the blank area where it thinks monitor 2 should be, then the window on screen slides off monitor 1 and disappears.
I can send you a screenshot if it helps to explain but do not know how to incorporate it with this message