[XeTeX] problem with discretionary

[jfbu]

Hi,

I need some help to identify which XeTeX release fixed 
that problem, the mwe is

\catcode`@ 11
\XeTeXinterchartokenstate=1
\newXeTeXintercharclass\french at punctthin 
\XeTeXcharclass `\; \french at punctthin
     \XeTeXinterchartoks 255 \french at punctthin = {\nobreak\thinspace}%
\catcode`;\active
\def;{\discretionary{\char`\;}{}{\char`\;}}
a;b
\bye

In  real life it appeared in a Polyglossia+French context
with the semi-colon make active to insert a \discretionary
similar to the above. There is no issue in lualatex.

It is currently seen at Python upstream (CPython) when
they try to build French docs (via Sphinx)

https://bugs.python.org/issue31589

and it would be nice to pinpoint which XeTeX release
precisely is ok. I know 0.99992 is bad and 0.99996 is good,
but can't easily bisect.

Best,

Jean-François