xelatex 下微縮字體和 Hoefler 文字的間距問題

Question

我在 XeLaTeX 下發現微字體和 Hoefler 文字有奇怪的間距問題。下面提供了一個最小的範例。當使用 xelatex 排版時，單字「he」和強調的單字「ought」重疊，如下所示：

如果我使用不同的字體（例如 Palatino），或刪除 microtype 包，甚至刪除第一句話，問題就會消失。任何關於導致此問題的原因的見解都非常感謝。

\documentclass{article}

\usepackage{microtype}
\usepackage{fontspec}
\setmainfont{Hoefler Text}

\begin{document}

For some cause, which I could not at the moment divine,
Arthur was unusually grave and silent during our walk home.
It could not be connected with Eric Lindon, I thought, as he
had for some days been away in London: so that, having Lady
Muriel almost ‘all to himself’—for \emph{I} was only too
glad to hear those two conversing, to have any wish to
intrude any remarks of my own—he \emph{ought},
theoretically, to have been specially radiant and contented
with life.

\end{document}

更新供參考：我正在使用目前版本的 TeX Live 2013、microtype (2.5a) 和 XeLaTeX (0.9999.3)，如下面的問題所述。