bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Eli Zaretskii]

> Date: Wed, 16 Oct 2019 22:37:40 +0200
> From: Ergus <spacibba@aol.com>
> Cc: Eli Zaretskii <eliz@gnu.org>, andreyorst@gmail.com,
>       37774@debbugs.gnu.org
> 
> >>> (custom-set-faces
> >>>  '(region ((((class color) (background light)) (:background "gray90"))))
> >>
> >> So maybe we should modify custom-set-faces to preserve the :extend
> >> attribute?  Would that solve the problem?
> >
> >I don't know how feasible this is.  This looks like a hack.
> >
> Actually it will be a transition workaround than could be removed in
> emacs 28 for those who don't want (or know) how to update manually
> because they made it with the interface.
> 
> But will break the case when the user explicitly wants a non extensible
> region face and set that in the custom-set-face section in his init.

If we implement this only when the custom-set-face does NOT include an
explicit :extend setting, then there will be no breakage, I think.