bug#74536: 30.0.50; undocumented box behaviour in faces

[Al Haji-Ali <abdo.haji.ali@gmail.com>]

The output of the following code is

--8<---------------cut here---------------start------------->8---
(progn
  (fundamental-mode)
  (insert (concat "\n"
                  (propertize "hello" 'face '(:box (:line-width (3 . 0) :color "red")))
                  (propertize " again" 'face '(:box (:line-width (3 . 0) :color "yellow")))
                  (propertize " other" 'face '(:box (:line-width (3 . 0) :color "green"))))
          "\n"))
--8<---------------cut here---------------end--------------->8---


|hello again other|

where the left border is red and the right one is green and the yellow border is completely hidden. Having negative line-width produces the same output.

It seems that one cannot have multiple adjacent borders unless one has some text in between without a border, like this
--8<---------------cut here---------------start------------->8---
(progn
  (fundamental-mode)
  (insert (concat "\n"
                  (propertize "hello" 'face '(:box (:line-width (3 . 0) :color "red")))
                  " "
                  (propertize " again" 'face '(:box (:line-width (3 . 0) :color "yellow")))
                  (propertize " othe " 'face '(:box (:line-width (3 . 0) :color "green"))))
          "\n"))
--8<---------------cut here---------------end--------------->8---

produces

|hello| | again other|

with (red, red, yellow, green) borders.

I find this behaviour, if intended, sometimes helpful as it allows one to have a one-sided border. Though it would be helpful if the exact mechanism is documented.

In any case, I was also originally expecting, and wanting, an output like this
|hello| again | other|

with `(red, yellow, green, green)` and no text in between, but it seems there is no way to achieve such output? Is that correct?

-- Al