[XeTeX] unicode-math and underbrace bug

[Daniel Greenhoe]

When I use the unicode-math package and the underbrace command, I get
sporadic errors in which the underbrace is (sporadically) shifted to
the right. This occurs for all the math fonts that I have tried which
include the following:
   xits-math.otf
   Asana-math.otf
   latinmodernmath-regular.otf
   texgyrepagellamath-regular.otf
   texgyretermesmath-regular.otf

Here is a minimal example:

\documentclass[oneside]{book}
%
 \usepackage{unicode-math}            % unicode math support
 \setmathfont{xits-math.otf}
 %\setmathfont{Asana-math.otf}
 %\setmathfont{latinmodernmath-regular.otf}
 %\setmathfont{texgyrepagellamath-regular.otf}
 %\setmathfont{texgyretermesmath-regular.otf}
%
\newcommand{\mcom}[2]{%
  \begin{array}[t]{c}%
    \underbrace{#1}\\%
    $#2$%
  \end{array}%
  }%
\begin{document}%
\[ \mcom{\sin^2x+\cos^2x=1}{a trig. identity} \]
\[ \mcom{\sin^2x+\cos^2x=1}{a trig. identity} \]
\[ \mcom{\sin^2x+\cos^2x=1}{a trig. identity} \]
...
\end{document}

Attached is a tex file and two sample output pdf files.

Can anyone help me out?

Many thanks in advance,
Dan
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