bug#37700: 27.0.50; undo mouse-drag-and-drop-region ineffective

[martin rudalics]

> Undoing mouse-drag-and-drop-region doesn't work:
>
> 1. (setq mouse-drag-and-drop-region t)
> 2. mark a region
> 3. use the mouse to drag the region elsewhere
> 4. Undo (twice)
>
> Result: the dragged text disappears from its destination, but does not 
reappear where it was dragged from.
> Instead, we get the "No further undo information for region" error.
>
> The action can be undone by moving point and backing up a different undo 
branch a few steps, but that is hardly ideal.

I suppose this the effect of undo working on the active region only.
A very nasty invention.  Try deactivating the region right after 3.

martin