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

[martin rudalics]

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?

martin