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

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Wed, 16 Oct 2019 17:27:05 +0200
> Cc: 37700@debbugs.gnu.org, tkk@misasa.okayama-u.ac.jp,
>         homeros.misasa@gmail.com
> 
> After trying various approaches, the attached patch looks somewhat promising. 
> It adds a new element type to buffer-undo-list, `unconfined', which disables 
> selective undo for one record. Consider it a proof-of-concept.
> 
> While it has the advantage of not requiring the user to change any settings, 
> I still prefer the first approach (an option to disable undo confinement), as 
> it's less intrusive and more generally useful. However, this should work as 
> well.

Thanks.  I don't consider myself an expert on undo, but the patch
looks reasonable to me.  I think we should describe this new element
in the ELisp manual, but other than that, I think it can go in.

Stefan, any comments?