[lltx] linebreak_filter problems

Frank Mittelbach frank.mittelbach at latex-project.org
Sun Jun 10 16:50:26 CEST 2012

Hi folks,

can somebody help me to identify if I do something wrong or if there is 
a bug with linebreak_filter.

Th problem is documented at length in


In a nutshell I try to replace (at certain times) TeX's linebreaking 
algorithm and replace it with a simple version that just packs up the 
hlist into an hbox and returns that.

If I do that the resulting box appears on the current vertical list (ie 
is seen via \showlists) but at the same time

  \lastnotetype  returns -1
  \lastbox       picks up a void box

ie to some parts of TeX the list appears to be empty. Here is an MWE

      function hpackparagraph  (head)
        return node.hpack(head)
      callback.register("linebreak_filter", hpackparagraph)



Any ideas what goes wrong or how I should alter things to make this 


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