(More from Frank Mittelbach on this subject - he is not subscribed yet but will
be ASAP)
I'm now fairly certain this this is a bug in the current implementation of
linebreak_filter. What seems to happen is the following: when TeX builds a list
(in this case a vertical list) it keeps track of it through a pointer to the
head and to the tail of that list. Now when a paragraph is broken into lines
those lines get appended to the current vertical list by the linebreaking
algorithm and the tail pointer is updated to point to the new end of this list.
However, if thelinebreak_filter replaces TeX's algorithm the material from that
filter is appended to the list but the tail pointer is not changed. Thus in my
example it still points to the node before the paragraph (which happens to be
nil as the paragraph starts the vbox).
Fortunately this tail pointer is accessible and modifiable from Lua code, so
there is a way to fix this. We can't do this in linebreak_filter, however,
because the current luatex code that calls the filter expects the tail in
precisely this wrong as it does some final checkup and changes.
But what we can do is to use the followup file post_linebreak_filter to do the
work for us, because by then all these things have happened. So the workaround
looks like this:
function hpack_paragraph (head)
h = node.hpack(head)
return h
end
function fix_nest_tail (head)
tex.nest[tex.nest.ptr].tail = node.tail(head)
return true
end
callback.register("linebreak_filter", hpack_paragraph)
callback.register("post_linebreak_filter",fix_nest_tail)
The second filter simply updates TeX's tail pointer in the current semantic
nest to become the tail of the result produced from the line breaking. A bit
nasty, but at least it allows to use thelinebreak_filter in the current
implementation.
If you wonder what this this code does: without anything further it changes the
output so that every paragraph is returned as a single line (and indentations
like from itemize or similar environments are dropped) and inserts like
footnotes or \vadjusts stay within those hboxes. So it needs a little more to
become useful, but let your imagination run.
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