Fixed fill-column-indicator face removing background.

*lisp/faces.el: Removed background in default fill-column-indicator
 face.
*src/xdisp.c: fill-column-indicator face merged with saved face.

doc/emacs/display.texi

@@ -1186,7 +1186,8 @@ initialization tries to set it to U+2502 or @sampl{|}.
 @item fill-column-indicator
 @vindex fill-column-indicator
 Specifies the face used to display the indicator.  It inherits its
-default values from shadow and the default face.
+default values from shadow but without background color.  To change
+the indicator color you need to set only the foreground color of this face.
 @end table
 
 @vindex indicate-buffer-boundaries

lisp/faces.el

@@ -2502,7 +2502,9 @@ unwanted effects."
 
 ;; 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."

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);
 	            }
 
@@ -20656,13 +20656,13 @@ extend_face_to_end_of_line (struct it *it)
 	    {
 	      if (it->current_x == fill_column_indicator_column)
 	        {
-		  const int saved_face = it->face_id;
+		  const int saved_face_id = 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_id);
 		  it->c = it->char_to_display =
 		    XFIXNAT (Vdisplay_fill_column_indicator_character);
 		  PRODUCE_GLYPHS (it);
-		  it->face_id = saved_face;
+		  it->face_id = saved_face_id;
 		  it->c = it->char_to_display = ' ';
 	        }
 	      else