[XeTeX] LetterSpace and line breaking

[Vadim Radionov]

Dear list memers,

I encountered a problem: setting LetterSpace parameter somehow breaks line
breaking mechanism. Consider the following example (in Russian); the 1st
variant without \mbox has obvious break point missed. (I'm aware  that
letter spacing is strongly discouraged in English texts, suppose that I
have to reproduce some ancient design as close as possible.)

What kind of bug is it?

Thank you in advance,
Vadim

\documentclass{book}
\usepackage{geometry}
\geometry{width=9.5cm}

\usepackage[no-sscript]{xltxtra}
\usepackage{polyglossia}

\setdefaultlanguage{russian}
\newfontfamily\russianfont[Mapping=tex-text,Script=Cyrillic]{DejaVu Serif}
% {PT Serif}

\emergencystretch4em

\begin{document}

3. {\addfontfeature{LetterSpace=40} Повествование} \ состоит из описания
истории предмета по
существу, т. е. как возник вопрос, который подлежит разрешению,
и как сложилось само дело.

3. \mbox{\addfontfeature{LetterSpace=40} Повествование} \ состоит из
описания истории предмета по
существу, т. е. как возник вопрос, который подлежит разрешению,
и как сложилось само дело.

\end{document}
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