[luatex] Trouble with the output routine?

[Paul Isambert]

There's a strange bug in LuaTeX v0.52 (but it also happened in v.51, and 
before that I can't tell). Here's a minimal example:

\output{\shipout\box255}

\topskip\baselineskip
\vsize3\baselineskip
\def\endline{\hfil\penalty-10000}

This is\endline
a 4-line\endline
paragraph\endline
that ends here\par

\vskip0pt
\hbox{A TITLE?}
A new paragraph...

\bye

(The \endline macro is just because I was too lazy to typeset an entire 
paragraph and is completely immaterial.)
So we have 3-line pages (which is immaterial once again). The 
interesting thing is that the second page should show:

that ends here
A TITLE?
A new paragraph...

but what happen is

A TITLE?
that ends here
A new paragraph...

with apparently no interline glue between the first two lines. This 
happens whatever the value of \vskip; interestingly, the skip (when 
non-0) appears after "that ends here". If your remove the skip, it works 
correctly again.
This happens with an old plain format without primitives enabled, and 
also with a luatex-plain format, in whatever output, with no callback 
anywhere, so apparently it's the engine. (And of course, it doesn't 
happen in pdfTeX.)

Best,
Paul