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 08:01:09 -0400
> Cc: 70637@debbugs.gnu.org
> 
> 
> 
> > On Apr 29, 2024, at 7:40 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> > 
> >> From: JD Smith <jdtsmith@gmail.com>
> >> Date: Sun, 28 Apr 2024 20:50:31 -0400
> >> 
> >> 
> >> I have identified a display bug related to placing :box face parameters within 'display strings.  Disable font-lock mode in *scratch* and evaluate:
> >> 
> >> (insert "\n"
> >> (propertize " " 'display (propertize "   " 'face '(:box t)))
> >> (propertize "middle" 'face '(:box t))
> >> (propertize " " 'display (propertize "   " 'face '(:box t))))
> >> 
> >> So far so good: the box is correctly merged across all three elements and wraps around them.  But now move point within and across the displayed boxed text.  Internal vertical divisions bars separating the 'display and normal :box regions appear.  
> >> 
> >> If you add a face box property to the blank flanking strings as well, this prevents the internal boundary from appearing.
> > 
> > How important is it to fix this use case (as opposed to using the
> > workaround you describe in the last sentence)?  The price for fixing
> > it would be that we will need to redraw more than the single glyph
> > below the cursor when showing the cursor (which with the default
> > blink-cursor-mode happens twice a second), which will cause flickering
> > around the cursor, and I wonder whether it's justified?
> 
> The workaround may be enough for most cases, unless you can't apply an overall 'face for some reason.

Why would we be unable to apply the box face on characters that are
not actually shown?

> Any sense why it leaves the vertical bars behind?

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.