[XeTeX] Unexplained behavior

[Bogdan Butnaru]

Hello! I encounter funny behavior when compiling a document; small
example below my signature. If you look very carefully, the line that
begins with \sepdash is typset slightly farther away from the line
above it than the other two lines are.

I have a custom macro for typesetting em-dashes with a bit of space;
it’s longer than the one in the example, but this is the shortest that
still exhibits the problem. Note that if a simple em-dash is used, or
if the spaced one is not at the start of a line, the line is not
spaced more than usual. The extra space is subtle, but I’m typesetting
a document with an exact number of lines on the page and it complained
of an underfull vbox.

I’m very curious why this happens; nothing I know about TeX suggests
what’s the problem. (I’d also like a solution if possible, but it’s
not that much of a big deal, I can use a different macro to
work-around the problem.)

-- Bogdan Butnaru

\documentclass[final]{memoir}
\usepackage{xunicode}
\usepackage{fontspec}

\def\emdash{—}
\def\sepdash{\thinspace—\thinspace}

\begin{document}
Aaaaaaaaaaaaaaaaaaaaaaaaaaa abcdef abcdef abcdef abcdef abcdef abcdef
abcdef abcdef.

\emdash{}abcdefabcdefabcdefabcdefabcdefabcdef

Aaaaaaaaaaaaaaaaaaaaaaaaaaa abcdef abcdef abcdef abcdef abcdef abcdef
abcdef abcdef.

\sepdash{}abcdefabcdefabcdefabcdefabcdefabcdef

Aaaaaaaaaaaaaaaaaaaaaaaaaaa abcdef abcdef abcdef abcdef abcdef abcdef
abcdef abcdef.

a\sepdash{}bcdefabcdefabcdefabcdefabcdefabcdef
\end{document}