[XeTeX] Discretionary hyphens don't work in paragraphed footnotes

[David Carlisle]

Here is a plain tex example, not quite as minimal as I'd like but out
of time for now.

This hyphenates with pdftex but not xetex

\nopagenumbers

\catcode`\@=11

\ifx\Umathchar\@undefined
\font\x=cmr12
\else
\font\x="[lmroman12-regular]:mapping=tex-text"
\fi

\x



\newbox\FN at tempboxa
\newbox\FN at tempboxb
\newbox\FN at tempboxc

  \long\def\makefootnoteparagraph{\unvbox0 \makehboxofhboxes
    \setbox\FN at tempboxa=\hbox{\unhbox\FN at tempboxa \removehboxes}
    \noindent
    \unhbox\FN at tempboxa\par
  }
  \def\makehboxofhboxes{\setbox\FN at tempboxa=\hbox{}%
    \loop
      \setbox\FN at tempboxb=\lastbox
      \ifhbox\FN at tempboxb
      \setbox\FN at tempboxa=\hbox{\box\FN at tempboxb\unhbox\FN at tempboxa}%
    \repeat
  }
  \def\removehboxes{\setbox\FN at tempboxa=\lastbox
    \ifhbox
      \FN at tempboxa{\removehboxes}%
      \unhbox\FN at tempboxa
    \fi
  }


\setbox0\vbox{XXXXXXXXXX just a few normal words to fill up the line
  up to my x x x zzzzzz\-zzzzz}

\global\setbox\FN at tempboxc\vbox{\hbox{XXX}\makefootnoteparagraph}%
\tracingonline1\showbox\FN at tempboxc



\unvbox\FN at tempboxc

\bye