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

[Tak Kunihiro]

> But I also faintly remember a discussion with the author of
> 'mouse-drag-and-drop-region' on whether it is a good idea to activate
> the mark after dropping - IIRC he did have some argument in favor of
> it.  Maybe it would be easier to add an option for not activating the
> region when dropping than adding one for 'undo'.

On revision of writing, I often want to move a sentence around to fit
into right place, without loosing the sentence from sight.  Most of the
time I cannot relocate the sentence to the best place by single
drag-and-drop operation thus I want to maintain region active.

I found that I did not notice problem pointed by Mattias because I
assign `undo' by `redo+.el' to C-/.  I agree that undo behavior by C-/
with Emacs -Q is inconvenient!

https://www.emacswiki.org/emacs/redo+.el