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

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Thu, 31 Oct 2019 12:00:08 +0100
> Cc: Eli Zaretskii <eliz@gnu.org>, 37700@debbugs.gnu.org,
>         tkk@misasa.okayama-u.ac.jp, homeros.misasa@gmail.com
> 
> > But now that I think about it, maybe a better option would be to check
> > 
> >    (when (symbolp last-command)
> >      (get last-command 'undo-inhibit-region))
> > 
> > and then put the `undo-inhibit-region` property on
> > `mouse-drag-and-drop-region`.
> 
> Thank you, this looks like the best idea so far. A very simple change, yet 
> effective in practice. Not perfect --- last-command is not buffer-local, and 
> even switching to a different frame and back will change it --- but good 
> enough.
> 
> Patch attached.

Fine with me (I proposed something like this during the original
discussion), but please document this property in the ELisp manual.

Thanks.