Microsoft Excel seems to have trouble with correctly applying the mark feature for Estedad. Apparently, it cannot properly handle the decomposed structure.(There is no issue with MS Word or MS PowerPoint)
Expected:
One thing I’ve noticed is that the decomposed structure works without any issues in some other Arabic fonts (for example, Dabestan from MaryamSoft). So there is hope that a workaround or solution exists. any insights would be appreciated.
The dots shouldn’t be mark. Are you decomposing sheen-ar into seen-ar and three dots?
Yes, I mentioned it. It uses a decomposed approach (the input is decomposed into base and mark glyphs (strokes and dots), instead of using precomposed glyphs built from components of stroke and dots).
MaryamSoft has been using this method for years, moving dots by means of kerning them with other invisible characters. I use the same approach to change dot forms via stylistic sets here.
It works everywhere else, and Excel works fine with the MaryamSoft font. So there seems to be some kind of priority or flag causing this issue.
Not sure at all about Excel. A lot of things do not work there. Wouldn’t be surprised if what you see in other fonts are the precomposed fallbacks.
GPOS work in MS Office apps, add an empty fvar table. There is a script called winfix.py in the Post Production subfolder of the mekkablue scripts. You can run it in the Terminal with python3 winfix.py "~/Desktop/My Fonts/*.ttf" (the part between straight quotes should be the path to your TTFs.update:
fvar table.All MaryamSoft fonts are static, so it seems Excel indeed has an issue with variable fonts that use a decomposed structure.
I’m not sure if there’s a specific person or team at Microsoft I could reach out to about this. if you know, please let me know.
Does it work in Excel for Windows? I think it has more to do with with the text engine in Excel for Mac that cannot process the ccmp properly.
It does work for me while editing the text, at least under certain circumstances:
But once I click out of the cell, it appears to disable GPOS:
But the issue is definitely more complex, because it would also snap back sometimes while editing, but I have not been able to reproduce it reliably.
For the time being I would not trust Excel for Mac with variable fonts, nor with GPOS. I see similar issues with Chromium-based browsers. It works remarkably well in TextEdit (and presumably all CoreText-based apps).
Thanks for the further tests. I tested it on Windows as well, it behaves the same as on macOS.
It seems to be a general issue with glyph positioning an even substitution. Almost all fonts are affected, regardless if they are decomposed or not, though in many decomposed fonts the issue is more pronounced.
Even Callibri is affected. Left while editing the text and right while not. The mark positions are slightly different, but also the beh-like glyphs are different ones:
I informed my contact at Microsoft. Now we need to wait. 
That’s great, thank you! :))
The Calibri example is interesting. The beh teeth are all done in calt as precomposed alternates, while the seen yeh is a skeleton ligature with dots placed via OT.
