[XeTeX] Arabic ligatures

[Musa Furber]

I started playing around with Arabic in XeLaTeX.

Geeza Pro works well, though it is not close enough to nuskhi (the 
model for printed books) or thuluth (the model for Qur'an) for my 
tastes.

Al Bayan Plain is closer. Unfortunately, something odd is happening 
with the ligatures.


Compare this sample using Geeza Pro:
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with the same using Al Bayan Plain:
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Notice that in the Geeza sample, there is an alif. It's the vertical 
line to the left of the blank space between the two words. That alif is 
missing in the Al Bayan sample.

I checked the sample in TextEdit, which takes advantage of typography 
gobbledygook. The same thing happened here as with XeLaTeX.

I then checked the sample in Mellel, which does not seem to use 
typography gobbledygook. There, Al Bayan did not lose its alif.

So, the problem is not XeLaTeX.

Can anyone suggest a solution around this?

Regards,
Musa