[XeTeX] Placement of tanwin for indefinite accusatives in Arabic, arabxetex

[Nathan Camillo Sidoli]

I have a question about the placement of tanwin for indefinite 
accusatives in arabxetex. (And a related question for Amiri font.)

My understanding in that the preferred style in Arabic is that the 
tanwin should be placed over the final consonant, even if there is a 
final alif.

This was the default in the original ArabTeX, and could be turned off 
with \newtanwin.

Here is a minimal example (minimal_tanwin.pdf):

\documentclass{article}
\usepackage{arabtex}
\vocalize

\begin{document}

\<_h.taN> $\quad$ \<m_tl_taN>

\end{document}

But with arabxetex this seems not to be the default, and instead I get 
the tanwin above the final alif, as is done in Persian.

Here is an example (minimal_tanwin_xelatex.tex), which also shows how 
the placement can be done manually with unicode:

\documentclass{article}

\usepackage[novoc,fdf2noalif]{arabxetex}
\newfontfamily\arabicfont[Script=Arabic,Scale=1.4]{Amiri}
\newcommand{\A}{\textarabic}

\begin{document}

\A{_h.t"aN} $\quad$ \A{m_tl_t"aN}

\bigskip

\A{خطًا}  $\quad$ \A{مثلثًا}

\end{document}

How can I get the correct Arabic behavior with arabxetex? Also, why is 
the tanwin placed so high when I use unicode and Amiri? Is there a way I 
can pull it down closer to the consonant?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: minimal_tanwin.pdf
Type: application/pdf
Size: 11631 bytes
Desc: not available
URL: <http://tug.org/pipermail/xetex/attachments/20150424/e23ffcd3/attachment-0002.pdf>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: minimal_tanwin_xelatex.pdf
Type: application/pdf
Size: 10532 bytes
Desc: not available
URL: <http://tug.org/pipermail/xetex/attachments/20150424/e23ffcd3/attachment-0003.pdf>