Re: Question about display engine

[Eli Zaretskii]

> Date: Wed, 28 Aug 2019 20:19:58 +0200
> From: Ergus <address@hidden>
> Cc: address@hidden, address@hidden
> 
> >> 1) Base face sets background and extend; and face sets only background.
> >
> >The background of the base face will be extended.
> >
> 
> But AFAIU the actual merging rules now will create a new face (if not
> there already) that will have the new background

No.  At the point which we are discussing, only background of faces
which have the :extend bit set get merged.  So the background of a
face without that bit will simply not get merged.

You will probably need to write a variant of merge_face_ref (or add an
extra argument to the existing function) such that background color
attribute of a face is not merged unless the :extend bit of the face
is set; and similar for underline and other relevant attributes we
discussed.