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

[Eli Zaretskii]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Thu, 17 Oct 2019 17:08:39 +0300
> 
> > With Firefox these diffs are boxed in a subarea of the Firefox window.
> > They do not start or extend at the window edges and text in these
> > boxes is static, can neither overflow into a newline nor be broken.
> 
> I think we can think of this "subarea" as directly corresponding to an 
> Emacs window for the purposes of this discussion, when we think how 
> diff-mode should display things.

If you select text in Firefox, what do you see? does it extend the
selection to the edge of the window?