Strange chapter influence on quotation environment

Question

The code below give a strange result when I comment out the \begingroup end \endgroup: the quotation environment has different vertical space to the text before ("Some text").

What is it that chapter defines that the quotation environment behaves different?

\documentclass{book}
\parindent 0pt
\begin{document}

%\begingroup  % <--- remove 
\chapter{abc}
%\endgroup    % <--- remove

Some text

\begin{quotation}
quotation with(out) vertical distance to the previous text
\end{quotation}

\end{document}

Answer 1

LaTeX section headings locally redefine \everypar to remove indentation in the first paragraph so if you put them in a group things will go wrong.

Also, in particular it causes \if@nobreak to be true rather than false just before the quotation, which makes the latex list code take a different path.