Hi Werner,
According to the table on page 6 in the current microtype package,
XeTeX *does* support character protrusion!
I said that in my first mail. XeTeX supports protrusion, but
(unfortunately) not expansion. In my experience, protrusion has little
effect; expansion is where the interest lies.
Karl, can you please add that to your patch?
Thus, I don't think it's worth the trouble to add support when it's only
protrusion. So I won't be working on this. Sorry.
Having a uniform behaviour for all major TeX
I believe texinfo.tex still supports Knuth's original "tex", which of
course does not have any microtype support, though maybe that does not
count as a "major engine" nowadays :).
BTW, separate from any microtype questions, at one time LuaTeX could
have different output (line breaking) from the other engines in obscure
cases. I don't know if that's still true, but I suspect so. I've
forgotten the details, unfortunately. Nothing to do about it, anyway.
--best, karl.
