Potential issues with this :

• Kashidas inserted between contextual alternatives (which are heavily used in Arabic fonts) break the shaping. Unless you intend to run the shaper afterwards.
• The example given is ugly looking. Kashida shouldn't be used this heavily.
• some Arabic fonts make use of justification alternates, which look better but are not widely supported. (only in indesign iirc). but even indesign just applies to EVERYTHING which looks as bad as the example photo.

The naive implementation for kashida insertion shouldn't be the default behavior (for line lengths this long it is almost always better to just stretch and shrink spaces). If it should exist at all it should be an opt-in toggle.

Here is a three-part essay about Arabic justification by Titus Nemeth: https://research.reading.ac.uk/typoarabic/on-arabic-justification-part-1/ which goes over the history and the issues. You'll find one of the historical findings is that native typesetters prefer to stretch spaces and use alternate shape forms rather than inserting kashidas everywhere. kashidas are at best used very sparingly.

Edit : OG post has been edited and my comment was on the pre-edit screenshot. the edit took in some of comments which is appreciated.

I created a (n opinionated) crate to help with this: https://github.com/asibahi/kashida . Maybe you guys can find it useful. It has some custom rules for Arabic and Syriac, but also a generic function for other scripts like N'Ko and Adlam (using Unicode properties!)

While I believe using variable fonts to be the superior justification strategy, this is a useful fallback, before adjusting spaces.

Thanks @asibahi!

@asibahi that is awesome work!!