Re: Question about display engine

[Ergus]

On Thu, Aug 08, 2019 at 03:05:37PM +0200, martin rudalics wrote:
> > Yes, that's exactly the point. The only face I see that needs to be
> > extended so far is the region. If only the region is extended
> > (assuming we won't stop extending that one too) you won't need your
> > workarounds, extra settings, another flag in the face structure, or
> > call extend face to end of line most of the time.
>
> I'm afraid things are not that simple.  We have at least the secondary
> selection and 'hl-line-mode' to take care of.  Moreover, there might
> be users who do prefer the current way of extending (and not
> extending) faces to window edges.  And I have no idea whether image or
> rectangular regions require special treatment too.
>
> martin
You are right, I ignored those use cases, but I still don't think that
the faces are the right place to flag that. The line extension maybe
needs to be decided based on another text property. Maybe there are
already some conditions we can check dynamically. Because adding a flag
is a bit error prone when there are already some conditions.

There is also the case when the face to use comes from FACE_FOR_CHAR or
another is merged over that. Or when there is a highlight inside the
region at the end of the line text... That, in the display engine, I am
not clear yet how are handled.

About the user preferences I think it needs to be accepted the Eli's
choice (I will favor the uniformity+simplicity over over-specification,
but I am not very "emacsy" in that point). Because we won't make happy
everyone in any case.