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

[martin rudalics]

>> 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.

I miss you here.  Emacs now by default also extends the region to the
right window edge.  OTOH both Mozilla and Thunderbird here extend the
region to EOL only.

> 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

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.

But I think that our (e)diff blocks are affected by the change and all
their face settings probably have to change, as well as tables and
listings.

martin