[Tex/LaTex] Parentheses differ (XeLaTeX, fontspec, newtxmath, libertine)

fontspeclibertinenewtxxetex

After Internal error: bad native font flag I stumbled upon another weird problem with the combination XeLaTeX/fontspec/Libertine/newtxmath:

\documentclass{scrartcl}
\usepackage[no-math]{fontspec}
\usepackage{libertineotf}
\usepackage{amsmath}
\usepackage[libertine]{newtxmath}

\begin{document}
    \begin{equation}
              ( \frac{1}{1}       )
         \left( \frac{1}{2} \right)
         \bigl( \frac{1}{3}  \bigr)
         \Bigl( \frac{1}{4}  \Bigr)
        \biggl( \frac{1}{5} \biggr)
        \Biggl( \frac{1}{6} \Biggr)
    \end{equation}
\end{document}

The up-scaled result is:
http://img225.imageshack.us/img225/5576/bracketsv.png

It seems to me that the Libertine font is used for ( and and CM for ).
Compare the closing parentheses from the above image to the following where the fontspec package was not used:
http://img94.imageshack.us/img94/2015/brackets2.png
The parentheses have the correct size but again CM is used for the numbers.
pdfLaTeX and the libertine package produce the same output as XeLaTeX without fontspec.

Best Answer

This seems to depend on Linux Libertine, that gets in the way with \left( (which is implicitly used also in the \big commands), so that \left( chooses the parenthesis from Libertine and not from the newtxmath defined font.

Other fonts instead of Linux Libertine don't exhibit this issue.

A workaround. Add the following lines after the definition of the fonts:

\DeclareSymbolFont{parenthesis}{OT1}{ntxr}{m}{n}
\DeclareMathDelimiter{(}{\mathopen}{parenthesis}{"28}{largesymbols}{"00}
\DeclareMathDelimiter{)}{\mathclose}{parenthesis}{"29}{largesymbols}{"01}

We are explicitly requesting that the parentheses are chosen from the newtxmath font. This gives another issue, though: the parentheses in math mode (normal size) will be different from the text mode ones.