bug#40687: Missing right border on composed text used in 'display property

[Stephen Berman]

On Fri, 17 Apr 2020 15:44:36 -0400 Clément Pit-Claudel <cpitclaudel@gmail.com> 
wrote:

> Hi all,
>
> With the following sample code, I observe the results shown in the attached 
> image.  The first two "ab" have a border, but the last one only has 
> three-quarters of its border: the right side is missing.
>
> (defface my-button
>   '((t :box(:line-width -4 :style released-button)
>        :background"lightgrey":foreground"black"))
>   "Button face")
>
> (with-current-buffer (get-buffer-create "button")
>   (insert "\n")
>   (insert (propertize "ab" 'face 'my-button))
>   (insert " ")
>   (insert (propertize (compose-chars ?a '(Br . Bl) ?b) 'face 'my-button))
>   (insert " ")
>   (insert (propertize "ab" 'display (compose-chars ?a '(Br . Bl) ?b) 'face 
> 'my-button))
>   (insert " ")
>   (pop-to-buffer-same-window (current-buffer)))

I see the same thing, but I noticed that when you add a space to
composed characters of the display property, ie.:
 (compose-chars ?a '(Br . Bl) ?b ? )
then right side border appears.

Steve Berman