The following snippet contains the ingredients that can be used to ultimately fix the problem described on the emacs-devel mailing list beginning at:

https://lists.gnu.org/archive/html/emacs-devel/2018-01/msg00466.html

In a nutshell, it->pixel_width and it->current_x are both incorrect in that situation.  Because the X is wrong, all subsequent references to it->current_x on the same line are also wrong.  In this snippet, we create a new gizmo in dispextern.h called my_pixel_width so as not to break anything while working on this issue.  Someone more knowledgeable than myself (e.g., Eli) will need to figure out what other adjustments in x_produce_glyphs are necessary so that it->pixel_width == it->my_pixel_width in this particular situation.  It is a little confusing, but here is what happens in this snippet:

it->pixel_width "should be" equal to it->my_pixel_width.  If we do that, however, then it->current_x will be wrong.

it->current_x "should be" equal to it->current_x less one (1) font->space_width.  Setting it->pixel_width to be one (1) font->space_width less than what it was fixes the value of it->current_x and does not break anything else in the process (as far as I can see).


dispextern.h:2590

  int my_pixel_width;


xdisp.c:28298

  if (it->char_to_display == '\t'
      && !NILP (Vdisplay_line_numbers)
      && it->w->hscroll > 0
      && it->current_x < it->lnum_pixel_width)
    {
      int my_tab_width = it->tab_width * font->space_width;
      int my_x = it->current_x + it->continuation_lines_width;
      int my_next_tab_x = ((1 + my_x + my_tab_width - 1) / my_tab_width)
                          * my_tab_width;
      if (my_next_tab_x - my_x < font->space_width)
        my_next_tab_x += my_tab_width;
      if (!NILP (Vdisplay_line_numbers))
        my_next_tab_x += it->lnum_pixel_width
                         - ((it->w->hscroll * font->space_width)
                            % my_tab_width);
      it->my_pixel_width = my_next_tab_x - it->lnum_pixel_width - font->space_width;
      it->pixel_width -= font->space_width;
    }
    else
      it->my_pixel_width = 0;