bug#70637: :box vertical bar artifacts at 'display boundaries

[Eli Zaretskii <eliz@gnu.org>]

> From: JD Smith <jdtsmith@gmail.com>
> Date: Mon, 29 Apr 2024 11:19:52 -0400
> Cc: 70637@debbugs.gnu.org
> 
>  This happens when the glyph under cursor has the beginning-of-box or
>  end-of-box flag set.  When we display the entire stretch of characters
>  on that line, we (correctly) don't pay attention to these flags in the
>  middle of the glyph sequence, but redrawing the cursor draws just one
>  glyph, and knows nothing about those before or after it.  So it draws
>  the unnecessary border, because the glyph under cursor has the flag
>  set.
> 
>  Those box flags are set on the glyphs produced from the display
>  strings because when we process the beginning or end of the string, we
>  don't have any idea whether the characters of the underlying buffer
>  text before/after the string have the same value of the :box face, so
>  we cannot avoid setting these flags at the first and the last
>  character of the display string.
> 
>  
> I see, makes sense.  So the cursor blink code would also have to "look ahead/behind" the underlying glyph to
> know whether to ignore the flag.

It's not just to "look", it's actually to redraw.  because the logic
which determines whether we draw the borders lives in the code that
draws the glyphs on the glass, and to DTRT it needs to be presented
with a sequence of glyphs that begins before the one under cursor and
ends after it.

> Probably this is such a rare case that unless there are other related
> artifacts, it's worth documenting but not fixing.

Suggestions for how to document this are welcome.