Re: Fill column indicator functionality

[Alp Aker <alptekin.aker@gmail.com>]

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>> 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?

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).

This has two advantages: (1) By default, we display the indicator using the
existing background, but the user can still override by explicitly setting
the background attribute of the indicator face.  (2) This handles the
general case of a non-default background, not just region highlighting.
(See the attached screenshot for an example of the issue not involving
region highlighting.)

diff --git a/src/xdisp.c b/src/xdisp.c
index 1782748144..07c5ee0a1c 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -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);
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."

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