Very interesting. I just implemented a text shaper and renderer from scratch with support for complex scripts like Arabic, Nastaliq and Indic (will soon post about it here on HN). Now that you write about it, the lack of stretching really is a deficiency in the OpenType spec.
If you want a solution for this it has to happen in the rendering step, not the shaping (which is HarfBuzz's main task). The shaper has no information about the available space, but when rendering you could stretch individual glyphs to the desired width, similar to adjusting the width of whitespace in Latin, but more complex, because you actually have to modify the glyphs with a scale transform. I am not an expert on Arabic script by any means, but this should be possible IMO. It would at least be an interesting experiment.
the entire article has llm tells all over it. i read it anyway, and i'm grateful for all the facts i learned (although i cannot trust all of them, for reasons aforementioned), but i genuinely (!) think it's a shame because the topic is an absolutely fascinating one
(i will permit myself to not explain in excruciating detail why i feel that way about this, as we have this discussion several times a day on this site)
If you want a solution for this it has to happen in the rendering step, not the shaping (which is HarfBuzz's main task). The shaper has no information about the available space, but when rendering you could stretch individual glyphs to the desired width, similar to adjusting the width of whitespace in Latin, but more complex, because you actually have to modify the glyphs with a scale transform. I am not an expert on Arabic script by any means, but this should be possible IMO. It would at least be an interesting experiment.
(i will permit myself to not explain in excruciating detail why i feel that way about this, as we have this discussion several times a day on this site)
The hard part is that typography, shaping, bidi behavior, font fallback, search, and the editor model all leak into each other.
You cannot fix one layer cleanly when the assumptions are wrong in all of them.