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

[Philip Taylor]

Jonathan Kew wrote:

> Yes, it's very possible this is an engine bug. What would be most useful
> here, I think, would be a reduced (non-LaTeX!) test case demonstrating
> the underlying incorrect behaviour at the (Xe)TeX primitive level. Any
> takers?

Way beyond my abilities but I can demonstrate a probably-related
behaviour without using fontspec :

\documentclass[a4paper]{article}
\font \mainfont =
"[/TeX/Live/2014/texmf-dist/fonts/opentype/public/lm/lmroman10-regular]"
\mainfont
\usepackage[para]{footmisc}
\begin{document}
\footnote{{\mainfont XXxxXX just a few normal words to fill up the line
  up to my word hy\-phe\-na\-te\-me please}}
\end{document}

This hyphenates after "hyphen", which it should not.  Package "footmisc"
does a great deal of \unhboxing; I wonder if it is losing the original
hyphenation points and then finding new ones ?

** Phil.