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From: Eli Zaretskii <eliz@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: Question about display engine
Date: Sat, 07 Sep 2019 10:37:07 +0300
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To: martin rudalics <rudalics@gmx.at>
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> Cc: eliz@gnu.org, emacs-devel@gnu.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Sat, 7 Sep 2019 08:52:56 +0200
> 
> When I set the :extend attribute of the default face, everything seems
> to work as expected.  The problem is that if, in the earlier mentioned,
> 
>        /* Update the faces id and extend.  */
>        it->face_id = new_face_id;
> 
>        if (FACE_EXTENSIBLE_P (new_face))
>          it->extend_face_id = new_face_id;
> 
> no extensible new face is found, the assignment fails and the old
> region background stays in place for the rest of the window.  So it
> seems that you have to make sure that _some_ default background is
> always assigned here in case no face applied has the :extend attribute
> set.

The default face should always be merged into the face used for line
extension, that goes without saying.