Describe the bug
The Abkhaz alphabet is based on the Cyrillic alphabet, but it also uses several additional unique letters that are not used in any other languages. These appear to be supported only partially in Libertinus. In particular, the italic variant seems to be missing for several of them.
Steps to reproduce
The letters that are used in Abkhaz, but not in Russian are:
ә ӷ ҕ ӡ қ ҟ ԥ ҧ ҭ ҳ ҵ ҷ ҽ ҿ ҩ џ
I don't see any obvious issues at all with ә ӡ ҷ ҽ ҿ ҩ џ. The letters ә, ҷ, and џ are also used in other languages (Kazakh, Tajik, Serbian). The letters ӡ, ҽ, ҿ, and ҩ are unique to Abkhaz, but as far as I can see, they are fine. That said, just to be on the safe side, it may make sense to verify that the font actually includes them all; it's possible that I just see fallback that happens to be good!
I see some relatively small issues with the letters қ and ҟ in both Roman and Italic variants, both capital and small. I might be wrong, but they appear to have some differences with their plain Russian counterpart letter к.
I see a huge issue with the letter ԥ, both capital and small. It appears not to be included in the font at all. It's a relatively recent addition to Unicode. It's the letter п with descender, similar to the descenders in ц, щ, қ, ҭ. Also note that its Italic variant is supposed to look similar to the Italic п.
I see a significant issue with the letter ҧ. I'd expect its Italic variant to look similar to the Italic п (more rounded), but it looks just square.
I see a significant issue with the letter ҭ. I'd expect its Italic variant to look similar to the Italic т (more rounded, three legs), but it looks just square and with one leg. In addition, its Roman capital variant also looks quite different from the plain Cyrillic Т.
I see a significant issue with the letters ӷ and ҕ. I'd expect their Italic variant to look similar to the Italic г (more rounded, like a mirrored s), but it looks just square.
I see a significant issue with the letter ҳ. I'd expect its Italic variant to look similar to the Italic х (more rounded), but it looks just straight.
I see a significant issue with the letter ҵ. I'd expect its Italic variant to look similar to a combination of the Italic т and ц, but it looks just straight.
I added a screenshot:
I took it on Ubuntu 21.04 in LibreOffice. You can see a comparison of all these letters in Libertinus Serif and Liberation Serif, which is mostly better in its coverage of Abkhaz (albeit not perfect). I included several letters from Russian and Ukrainian, which are related to the relevant Abkhaz characters, to show how they are supposed to be similar.
Describe the bug The Abkhaz alphabet is based on the Cyrillic alphabet, but it also uses several additional unique letters that are not used in any other languages. These appear to be supported only partially in Libertinus. In particular, the italic variant seems to be missing for several of them.
Steps to reproduce The letters that are used in Abkhaz, but not in Russian are: ә ӷ ҕ ӡ қ ҟ ԥ ҧ ҭ ҳ ҵ ҷ ҽ ҿ ҩ џ
I don't see any obvious issues at all with ә ӡ ҷ ҽ ҿ ҩ џ. The letters ә, ҷ, and џ are also used in other languages (Kazakh, Tajik, Serbian). The letters ӡ, ҽ, ҿ, and ҩ are unique to Abkhaz, but as far as I can see, they are fine. That said, just to be on the safe side, it may make sense to verify that the font actually includes them all; it's possible that I just see fallback that happens to be good!
I see some relatively small issues with the letters қ and ҟ in both Roman and Italic variants, both capital and small. I might be wrong, but they appear to have some differences with their plain Russian counterpart letter к.
I see a huge issue with the letter ԥ, both capital and small. It appears not to be included in the font at all. It's a relatively recent addition to Unicode. It's the letter п with descender, similar to the descenders in ц, щ, қ, ҭ. Also note that its Italic variant is supposed to look similar to the Italic п.
I see a significant issue with the letter ҧ. I'd expect its Italic variant to look similar to the Italic п (more rounded), but it looks just square.
I see a significant issue with the letter ҭ. I'd expect its Italic variant to look similar to the Italic т (more rounded, three legs), but it looks just square and with one leg. In addition, its Roman capital variant also looks quite different from the plain Cyrillic Т.
I see a significant issue with the letters ӷ and ҕ. I'd expect their Italic variant to look similar to the Italic г (more rounded, like a mirrored s), but it looks just square.
I see a significant issue with the letter ҳ. I'd expect its Italic variant to look similar to the Italic х (more rounded), but it looks just straight.
I see a significant issue with the letter ҵ. I'd expect its Italic variant to look similar to a combination of the Italic т and ц, but it looks just straight.
I added a screenshot:
I took it on Ubuntu 21.04 in LibreOffice. You can see a comparison of all these letters in Libertinus Serif and Liberation Serif, which is mostly better in its coverage of Abkhaz (albeit not perfect). I included several letters from Russian and Ukrainian, which are related to the relevant Abkhaz characters, to show how they are supposed to be similar.
Here's the LibreOffice file I used to create screenshot: ab-Libertinus-issues.odt