Re: Fill column indicator functionality

[Alp Aker]

> > User customization could be accommodated by something like the below (naive) patch.  We define
> > fill-column-indicator face to have unspecified background; when displaying the indicator we merge the
> > indicator face into the current it->face_id (the same face used for the stretch glyph preceding the indicator).
>
> What about the case there's no stretch, i.e. the indicator directly
> follows the last character of the line?

Apologies for my poor phrasing, but I think you might misunderstand the intent.  I meant that we use the background that is in effect before we draw anything related to the indicator, not that we take the face *from* the stretch glyph.  

I did miss that the indicator is also drawn in append_space_for_newline.  Revised patch below.

diff --git a/lisp/faces.el b/lisp/faces.el

index b933279064..207b627f25 100644

--- a/lisp/faces.el

+++ b/lisp/faces.el

@@ -2502,7 +2502,9 @@ line-number-current-line

 

 ;; Definition stolen from display-line-numbers.

 (defface fill-column-indicator

-  '((t :inherit (shadow default)))

+  '((t :inherit shadow :weight normal :slant normal

+       :underline nil :overline nil :strike-through nil

+       :box nil :invert-video nil :stipple nil))

   "Face for displaying fill column indicator.

 This face is used when `display-fill-column-indicator-mode' is

 non-nil."

diff --git a/src/xdisp.c b/src/xdisp.c

index 1782748144..61ea050b4a 100644

--- a/src/xdisp.c

+++ b/src/xdisp.c

@@ -20201,7 +20201,7 @@ append_space_for_newline (struct it *it, bool default_face_p)

       XFIXNAT (Vdisplay_fill_column_indicator_character);

             it->face_id =

       merge_faces (it->w, Qfill_column_indicator,

-                   0, DEFAULT_FACE_ID);

+                   0, saved_face_id);

             face = FACE_FROM_ID(it->f, it->face_id);

             goto produce_glyphs;

           }

@@ -20494,7 +20494,7 @@ extend_face_to_end_of_line (struct it *it)

                  XFIXNAT (Vdisplay_fill_column_indicator_character);

                it->face_id =

                  merge_faces (it->w, Qfill_column_indicator,

-                              0, DEFAULT_FACE_ID);

+                                    0, saved_face_id);

                PRODUCE_GLYPHS (it);

              }

 

@@ -20658,7 +20658,7 @@ extend_face_to_end_of_line (struct it *it)

          {

    const int saved_face = it->face_id;

    it->face_id =

-     merge_faces (it->w, Qfill_column_indicator, 0, DEFAULT_FACE_ID);

+     merge_faces (it->w, Qfill_column_indicator, 0, saved_face);

    it->c = it->char_to_display =

      XFIXNAT (Vdisplay_fill_column_indicator_character);

    PRODUCE_GLYPHS (it);