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

[Eli Zaretskii <eliz@gnu.org>]

> 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?