Re: Question about display engine

[Eli Zaretskii]

> Date: Wed, 7 Aug 2019 17:57:38 +0200
> From: Ergus <address@hidden>
> Cc: address@hidden
> 
> >Such a face will not be a fixed face, it will have to be recomputed
> >whenever the face of the text changes, right?  E.g., if the face of
> >the text specifies some color, you'd want this additional face to have
> >the same colors, right?
> >
> We don't use the face itself, just to merge with the previous glyph.
> 
> >So it doesn't seem to be a face that can be customized in the usual
> >sense.  We could let the users specify face attributes they don't want
> >to see in face extension, though.
> 
> Please look the proposed patch. It may need some improvements, but at
> least the functional part is a decent solution for all the issues in my
> opinion.

First, let's not mix this with the display-fill-column-indicator-mode,
let's keep the changes for these two separate, OK?

And second, I don't think I understand what we expect users to do with
this face's customization.  Suppose the user customizes this face to
set the :underline attribute, will the effect be reasonable?

I thought we wanted to let users determine which attributes will be
_reset_, not _set_.  But faces, as we use them normally, don't allow
resetting attributes.

And, of course, this leaves the more general problem I described in my
message: what is the right behavior for extending the face that
crosses a newline.  (Note that the display engine in general doesn't
know whether a face that doesn't end before a newline will or won't
continue on the next screen line.)

Thanks.