bug#28936: move_it_in_display_line_to returns MOVE_POS_MATCH_OR_ZV before ZV

[Eli Zaretskii]

> Date:  Sun, 22 Oct 2017 11:05:45 -0700
> From:  Keith David Bershatsky <esq@lawlist.com>
> Cc:  28936@debbugs.gnu.org
> 
>   SET_TEXT_POS_FROM_MARKER (start_text_position, w->start);
>   start_display (&it, w, start_text_position);
>   move_it_to (&it, PT, it.last_visible_x, it.last_visible_y - 1, -1, 
> MOVE_TO_POS | MOVE_TO_X | MOVE_TO_Y);
>   target_x = it.current_x;
>   move_it_by_lines (&it, 0);

Did you verify that it.current_x is zero after this line?  If not, you
will not get to the right X coordnate here:

>   rc = move_it_in_display_line_to_x (w, &it, target_x);