[luatex] letterspacing in LuaTeX

[taco]

Ulrike Fischer wrote:
> 
> I don't know. But I now also looked at box \z@ 
> 
> And there you get with luatex as first box:
> 
> [...]
> 
> So box0 contains in luatex at the start 2 hboxes and so needs one
> step more to get void than in pdflatex.
> 
> The additional box disappears (and also the errors) if I remove the
> \tolerance\@M from SOUL at doword:

This makes sense. The extra \whatsit followed by a skip introduces
a possible linebreak point that is chosen thanks to the \tolerance\@M.

The luatex engine should probably suppress this potential breakpoint,
so I will create a bug tracker for that, but that may not help soul.sty
because the \whatsit will remain.

Best wishes,
Taco