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

[JD Smith]

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.