bug#51550: 29.0.50; Customize Group INS buttons sometimes don't have a left box line

[Eli Zaretskii <eliz@gnu.org>]

> From: Po Lu <luangruo@yahoo.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  stephen.berman@gmx.net,
>   51550@debbugs.gnu.org
> Date: Mon, 27 Dec 2021 18:50:22 +0800
> 
> I think I found the bug, it's in display_line.
> 
> When the label at_end_of_line is reached, it->start_of_box_run_p can be
> false.
> 
> Afterwards, set_iterator_to_next is called, which reseats the iterator
> to the next line, but doesn't set it->start_of_box_run_p if the face is
> now different from the previous face and also has a box.
> 
> I think the solution is to save the face ID of the iterator after the
> call to extend_face_to_end_of_line, then compare it to the face after
> the iterator is reseated to the next line, and set
> it->start_of_box_run_p to true if that face is different and also has a
> box.

Can you show me a simple Lisp that would reproduce such a problem?  I
don't think I follow your description.  You are actually saying that a
screen line cannot begin with text that has a face with the box
attribute set?