Thanks for the report Jonathan, I've filed a bug in our system.

Andrew

-----Original Message-----
From: Jonathan Kew [mailto:[email protected]] 
Sent: lundi 19 août 2013 03:00
To: Behdad Esfahbod; Harfbuzz; Andrew Glass (WINDOWS)
Subject: shaping of U+06C2 [uniscribe bug]

A further harfbuzz-vs-uniscribe discrepancy that I'm seeing is the shaping of 
the Arabic-script character U+06C2. Although (AFAIK) this character is normally 
used only in final position, it is classified by the Unicode standard as 
dual-joining (see ArabicShaping.txt), and therefore causes any following letter 
to take a right-linking (final or
medial) form.

In older versions of Unicode, U+06C2 was classified as right-joining, and so 
did not affect the form of a following letter. It looks like this change (for 
consistency with the behavior of its canonical decomposition
<06C1 0654>) was made in Unicode 4.1.

Unfortunately, uniscribe still (eight years later, even on Win8) seems to be 
treating U+06C2 as right-joining (and fonts shipped with Windows support it as 
such, and lack initial/medial forms).

This means that users are liable to omit the space or non-joiner that should be 
included in a phrase such as 
<U+062F,U+0631,U+062C,U+06C2,U+062D,U+0631,U+0627,U+0631,U+062A>, as uniscribe 
interrupts the joining after 06C2 even when a letter follows. 
When such text is rendered by a Unicode 4.1-or-later system, unwanted 
cursive-joining will occur.

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