Hi

The version on github is outdated. I have a workaround for that backgrownd issue in a patch I sent to Eli (in this mailing list) on yesterday. Please try that one and check if you experience the same problems.
I will update the github code latter because i am doing some changes in the lisp interface now.

On 17 March 2019 18:28:22 CET, Alp Aker <alp.tekin.aker@gmail.com> wrote:
>>> In graphical interfaces the space after the line is always filled
>with
>>> the background color of the face of the last produced glyph, even if
>I
>>> reset it to the saved value after the generation. The only solution
>I
>>> found so far was to add an extra glyph after reset the face to the
>>> default (saved) value, but hopefully there is a better way?
>> I don't think I understand what face is "the face of the last
>produced
>> glyph".  is that the face of the indicator character, is that the
>> default face, or is that something else?  Can you show a screenshot?
>I noticed an issue with non-default backgrounds that span newlines,
>such 
>as happens with region highlighting. I believe I ran into what Ergus is
>
>describing while working on a fix (see (3) below).  The version of the 
>code I looked at was 9dcaa15e5a, from Ergus's Github repo.
>
>You can see the issue with non-default backgrounds in the attached 
>screenshots mode-off.png and mode-on.png; activating 
>display-fill-column-indicator-mode truncates the highlighting on each 
>line.  To fix:
>
>1. The stretch glyph needs to be drawn in the current face, not the 
>fill_column face.
>
>2. The default fill-column face should have an unspecified background 
>and it should be merged into the current face during display, not into 
>the default face.
>
>3. If the fill-column face specifies a background, we need to reset the
>
>face to the saved face after producing the indicator glyph. Here I
>found 
>it necessary to insert another display element in order for the face 
>change to take effect before the background is extended to the end of 
>the line. (I used a 0-width stretch glyph.)  Without that, the 
>fill-column face is used (see the attached c.png for a screenshot).  I 
>believe this need to add another display element at the end of the line
>
>is what Ergus was asking about.
>
>
>diff --git a/lisp/faces.el b/lisp/faces.el
>index 153e6a208f..6b9980a77f 100644
>--- a/lisp/faces.el
>+++ b/lisp/faces.el
>@@ -2504,7 +2504,7 @@ line-number-current-line
>
>  ;; Definition stolen from linum.el.
>  (defface fill-column
>-  '((t :inherit (shadow default)))
>+  '((t :inherit (shadow)))
>    "Face for displaying fill column indicator line.
>  This face is used when `display-fill-column-indicator-mode' is
>  non-nil.
>
>diff --git a/src/xdisp.c b/src/xdisp.c
>index 8ac4be8dc7..7c4f9889eb 100644
>--- a/src/xdisp.c
>+++ b/src/xdisp.c
>@@ -20416,21 +20416,23 @@ extend_face_to_end_of_line (struct it *it)
>            int stretch_ascent = (((it->ascent + it->descent)
>              * FONT_BASE (font)) / FONT_HEIGHT (font));
>
>-          it->char_to_display = 
>XFIXNAT(Vdisplay_fill_column_indicator_character);
>            memset (&it->position, 0, sizeof it->position);
>            it->avoid_cursor_p = true;
>-          it->face_id = merge_faces (it->w, Qfill_column, 0, 
>DEFAULT_FACE_ID);
>            it->start_of_box_run_p = false;
>            it->object = Qnil;
>
>            append_stretch_glyph (it, Qnil, stretch_width,
>                                 it->ascent + it->descent,
>stretch_ascent);
>
>+          it->char_to_display = 
>XFIXNAT(Vdisplay_fill_column_indicator_character);
>+          it->face_id = merge_faces (it->w, Qfill_column, 0, 
>saved_face_id);
>            PRODUCE_GLYPHS (it);
>
>+          it->face_id = saved_face_id;
>+          append_stretch_glyph (it, Qnil, 0, it->ascent + it->descent,
>+                             stretch_ascent);
>            it->position = saved_pos;
>            it->avoid_cursor_p = saved_avoid_cursor;
>-          it->face_id = saved_face_id;
>            it->start_of_box_run_p = saved_box_start;
>            it->char_to_display = saved_char;
>            it->object = save_object;
>@@ -20566,7 +20568,7 @@ extend_face_to_end_of_line (struct it *it)
>            if (it->current_x == fill_column_indicator_line)
>              {
>            const int saved_face = it->face_id;
>-          it->face_id = merge_faces (it->w, Qfill_column, 0, 
>DEFAULT_FACE_ID);
>+          it->face_id = merge_faces (it->w, Qfill_column, 0,
>saved_face);
>            it->c = it->char_to_display = 
>XFIXNAT(Vdisplay_fill_column_indicator_character);
>            PRODUCE_GLYPHS (it);
>            it->face_id = saved_face;