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

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Fri, 11 Oct 2019 19:18:51 +0200
> Cc: martin rudalics <rudalics@gmx.at>, 37700@debbugs.gnu.org
> 
> another [reason] is that almost all other operations that can be undone 
> either deactivate the region or take place wholly inside it, so there is no 
> reason for the user to care about that quirk.

So perhaps a better way to resolve this situation is to teach 'undo'
about drag-and-drop, so that it doesn't undo selectively immediately
after drag-and-drop?

IOW, the selective undo feature AFAIU assumes that the region is set
up by the user before invoking 'undo', and that is not so in this
case.