Just have a suggestion when using "screen as overlay" for desktop prototypes.
It would nice if one of the screen "position" choices was "mouse position". So the overlay screen could display where the mouse position is.
Perhaps it would need an anchor point too (top - above the mouse...right, left, bottom etc...).
Additionally there could be a sub option to have the screen follow the mouse until overlay is close (on click).
This would allow you to emulate drag and drop behavior in the prototypes.