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

[Eli Zaretskii]

> Cc: 37774@debbugs.gnu.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Wed, 6 Nov 2019 18:06:04 +0100
> 
> One surprising effect with recent master is that with emacs -Q
> evaluating
> 
> (custom-set-faces
>   '(font-lock-comment-face ((((class color) (background light)) (:background 
> "Beige" :foreground "Black")))))
> 
> makes the comment background extend to the end of the window.  This
> means that unless I explicitly supply :extend nil, the behavior is as
> with Emacs 26.  Is that intended?

No, of course it isn't intended.

Jimmy, are you looking into this?  It sounds like there's a difference
with processing the :extend attribute when its value is 'unspecified'
and when it's nil.  Because just adding

  (set-face-extend 'FOO nil)

for the faces named by Jonas and Martin makes the problem go away, and
the faces aren't extended, as I'd expect.

Let me know if you need help in debugging or resolving this.

Thanks.