Re: Fill column indicator functionality

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 04 May 2019 11:37:13 +0200
> From: Ergus <spacibba@aol.com>
> 
> It looks like we need only to keep the background colour also for the indicator.

Are you sure that's all?  And that some users won't want the indicator
face's background to prevail?

> I don't know if there is an easy way to do so and if it worth the effort.

Given a face_id, you can get the face struct with FACE_FROM_ID.  The
face struct has a member named 'lface', which is a vector of the
face's Lisp attributes.  Each attribute in the vector has a known
index.  So doing something like this (untested) should change the
background color of the indicator as requested:

  int region_face_id = lookup_named_face (NULL, it->f, Qregion, false);
  struct face *region_face = FACE_FROM_ID (it->f, region_face_id);
  Lisp_Object bg_color = region_face->lface[LFACE_BACKGROUND_INDEX];
  face->lface[LFACE_BACKGROUND_INDEX] =
    STRINGP (bg_color) : bg_color : Qunspecified);

assuming that 'face' used in the ASET line is the face of the
indicator, as you compute it in your branch.