[XeTeX] difficulty with ancient greek diacriticals in XeTeX with Gentium Plus

[Richard Cobbe]

This may be slightly OT, since I think it's a font problem rather than a
XeTeX problem, but I'm hoping someone here may be able to give me a few
pointers.  If not, please forgive the noise.

I recently switched from using Gentium
(http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=gentium) to
Gentium Plus to typeset some classical Greek text, and I'm now getting
different results when I use combining diacriticals in the XeTeX input.

I've attached a very small XeTeX example that demonstrates the problem,
along with the output I get.  (The .tex file is in UTF-8.)  The first line
in the document body, as indicated, uses a precomposed Unicode character;
the second line uses equivalent combining diacriticals.  At least, I
thought they were supposed to be equivalent; as you can see from the PDF,
the output is different.  In the PDF, the "precomposed" line is the desired
output -- the diacriticals are supposed to be stacked, not superimposed.

This same input file works fine (stacked rather than superimposed
diacriticals) if I switch back to Gentium, which suggests that the
difference is in the fonts, rather than in XeTeX.

It's much more convenient for me to use the combining diacriticals, for
various reasons that aren't all that interesting here.  Is there something
in XeTeX/fontspec I can do to make that input work again, or is this a font
problem?

MacOS 10.8.3, TeXLive 2012.

Thanks much,

Richard
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