Re: Question about display engine

[Eli Zaretskii]

> Cc: address@hidden, address@hidden
> From: martin rudalics <address@hidden>
> Date: Sun, 11 Aug 2019 10:14:33 +0200
> 
>  > I don't think this is workable, because of face merging.  The actual
>  > face used to display the last character on a screen line can (and
>  > frequently does) come from merging several faces, and there's no
>  > meaningful answer to the question: which face did this attribute come
>  > from?
> 
> Answering this question should not be the task of a (face agnostic)
> display engine.  The face merging algorithm would have to decide
> whether the :extend attribute of the winning face should cause an
> extension of any attribute specified by that face.

What is the "winning face" in this context?

>  > For a face merged from 2 or more faces defined via defface, how
>  > do you tell whether or not to extend it?  Thus, such delegation can
>  > only yield inconsistent behavior and more bug reports.
> 
> But there's only one face that decides whether and how the background
> shall be set and there's only face that decides that for underlining.

Not necessarily true: two or more faces could specify the same value
of a particular attribute, including background and underlining.  The
:extend attribute could be different in some of these faces.