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

[Eli Zaretskii]

> Cc: 37774@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Thu, 17 Oct 2019 19:50:12 +0300
> 
> On 17.10.2019 19:29, Eli Zaretskii wrote:
> > If you select text in Firefox, what do you see? does it extend the
> > selection to the edge of the window?
> 
> It does not.
> 
> After considering it a bit, I might like Firefox-like behavior for the 
> region personally.

That's what happened to me as well.  So I think people who are
claiming it's a breaking change might try running with the change for
a week or so, perhaps they will change their minds.

> Even so, it might be safer to only offer such option and not change
> it by default.

We did, for the 'region' face.

> This example was about how diff-mode behavior did/should look, though.

We could consider individual faces for making them extend by default.
But there's a more general claim in this bug report: that the change
will screw many unbundled packages out there; if that is true,
changing some faces in core is not a solution.