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

[Juri Linkov]

>> (It's a pity that the long discussion of this before the development
>> started went without any such objections from the people who are
>> regulars on emacs-devel.)
>
> Btw, please take into account that what that change caused is that
> Emacs now behaves like other applications in this regard.

I don't know what applications behave the same.  I tried different
editors that I could find (namely LibreOffice Writer and xed)
and all they extend highlighting of the selected region
to the window right edge, not to EOL.

Also I looked how other applications extend diff blocks, and e.g.
GitLab extends diff background colors to the window right edge,
not to EOL, for example,
https://github.com/emacs-mirror/emacs/commit/3d6075e3ee8c447f8974b37007a1b1ae1af8917c

However, no other application extends underlines to the window edge
as Emacs used to do, this was a plain bug that this change fixed.