\flushbottom vs. fixing position of chapter heading in book class

Question

Consider this MWE:

\documentclass[12pt%,oneside%
]{book}
\usepackage{setspace}
\setstretch{1.2}
\usepackage{lipsum}
\begin{document}
%\raggedbottom
\chapter{ABC}
See how this chapter heading is not in the same place as the following one? Try the oneside option of the book class, and the problem disappears!
\chapter{ABC}
\lipsum[1-3]
\end{document}

You'll note the positions of the first and second chapter headings are different. This is quite annoying when switching from one doublepage to another after a very short chapter.

How can I fix this without using \raggedbottom or oneside?

Answer 1

It's a bug, er, feature in the book class, where \par is used for separating “Chapter 1” from the title, but \parskip is not set to zero. The usual setting for \parskip is 0pt plus 1pt, in order for TeX to be able to obey \flushbottom.

Just correct the bad behavior:

\documentclass[
  12pt,
  openany,% just to be able to see both chapter heads side by side
]{book}

\usepackage{etoolbox}
\makeatletter
\patchcmd{\@makechapterhead}
  {\parindent}
  {\parskip=\z@skip\parindent}
  {}{}
\makeatother

\usepackage{setspace}
\setstretch{1.2}
\usepackage{lipsum}

\begin{document}
\chapter{ABC}

See how this chapter heading is not in the same place as the following one? Try the oneside
option of the book class, and the problem disappears!

\chapter{ABC}
\lipsum[1-3]
\end{document}

The setting \parskip=\z@skip is local, because LaTeX does the printing of the chapter head in a group, so it won't propagate to the text typesetting.