Linegoal in (scr)book

Question

Linegoal calculates the wrong remaining line width when using the book or scrbook document classes. This problem does not happen with article etc.

MWE:

\documentclass{book}
\usepackage{linegoal}
\begin{document}
\overfullrule=5pt % To show the overfull lines
some line \parbox[t]{\linegoal}{some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content}
\clearpage
some different line \parbox[t]{\linegoal}{some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content some parbox content}
\end{document}

Result on page 1:

Page 2 is fine:

I think I narrowed it down to occur when there are different \evensidemargin/\oddsidemargin.

Answer 1

OK, I figured it out after sinking way too much time into this...

tl;dr Fix

Add this to your preamble (after loading linegoal):

\makeatletter
\let\Z@E@linegoal\relax % undefine the property
\zref@newprop*{linegoal}[\linewidth]{\dimexpr
   \linewidth -\the\pdflastxpos sp
   +\ifodd\c@page
      \oddsidemargin
   \else\evensidemargin
   \fi
   +1in+\hoffset
   \relax
}% linegoal zref-property
\makeatother

Note that this may lead to an endless loop of changed labels if a page break depends on a \linegoal and the \linegoal itself is moved across pages due to this. In such cases, you may have to adjust the page breaks manually.

Background

Basically, linegoal uses an internal counter \LNGL@unique that is increased by one at every \linegoal occurrence. The current value of this counter is used to create labels (in the aux file) that look like this:

\zref@newlabel{linegoal/posx.3}{\page{35}\posx{19599809}}
\zref@newlabel{linegoal.3}{\linegoal{\dimexpr \linewidth -19599809sp +\evensidemargin +1in+\hoffset \relax }}

In this example, the value of \LNGL@unique was 3.

The example also already hints at the problem: Page 35 is an odd page, yet we see an \evensidemargin. However, the linegoal package doesn't just assume \evensidemargin everywhere; it checks whether the page number is even or odd and uses the corresponding margin:

   \ifodd\zref@extractdefault{linegoal/posx.\the\LNGL@unique}{page}\c@page
      \oddsidemargin
   \else\evensidemargin
   \fi

... to get the number of the page on which a \linegoal is located, linegoal uses \zref@extractdefault{linegoal/posx.\the\LNGL@unique}{page}\c@page, i.e., if there is a label linegoal/posx.<counter value> in the aux file, then it extracts the page from there, otherwise it use the current page counter value (\c@page).

However, for some reason, the current value of \LNGL@unique is almost always wrong,¹ and hence, the returned page number is wrong (from the second LaTeX run onwards).

¹: You can try for yourself by editing a copy of linegoal.sty and, e.g., replace the code snippet shown above by

   \ifodd\zref@extractdefault{linegoal/posx.\the\LNGL@unique}{page}\c@page
      \oddsidemargin + \the\LNGL@unique pt
   \else\evensidemargin + \the\LNGL@unique pt
   \fi

then look at the produced labels in the aux file.

The Fix

Instead of relying on \zref@extractdefault (and thus the apparently wrong \LNGL@unique value), the proposed fix always uses the page counter.