Extraneous empty line within memoir's list environment (for very tight lists) if used with memoir's Spacing environment (for single-spacing)

Question

In order to typeset lists with very tight interline spacing, I normally define my own tightlist using memoir's list environment.

Even though my definition (see shortly) (1) ensures that all vertical spacing parameters are zero and (2) uses the Spacing environment to ensure single-spacing (in a 1.5-or-wider spacing environment), I strangely get an extraneous empty line after a nested tightlist:

\documentclass{memoir}

\newenvironment{mytightlist}[3]{%
  \begin{Spacing}{1}%
  \begin{list}{#1}{%
    \setlength{\topsep}{0pt}%
    \setlength{\parskip}{0pt}%
    \setlength{\parsep}{0pt}%
    \setlength{\itemsep}{0pt}%
    \setlength{\leftmargin}{#2}%
    \setlength{\labelwidth}{#3}%
    \setlength{\labelsep}{0.5em}%
  }%
}%
{\end{list}\end{Spacing}}


\begin{document}

Text.

\begin{mytightlist}{\(\bullet\)}{1.5em}{0.5em}
\item Text.
  \begin{mytightlist}{\(\circ\)}{1.5em}{0.5em}
  \item Text.
  \item Text.
  \end{mytightlist}% there is an empty line here that shouldn't be there
\item Text.
\end{mytightlist}

Text.

\begin{mytightlist}{\(\bullet\)}{1.5em}{0.5em}
\item Text.
  \begin{mytightlist}{\(\circ\)}{1.5em}{0.5em}
  \item Text.
  \item Text.
  \end{mytightlist}\vspace{0pt}% this magically prevents the empty line from occurring
\item Text.
\end{mytightlist}

Text.

\end{document}

Issuing \vspace{0pt} after the nested tightlist eliminates the empty line, so that solves the problem, but why does this happen in the first place, and why does this solution work? It seems like this behavior is caused by how the Spacing environment interacts with the list environment.

Answer 1

Don't use Spacing to begin with. But if you do (and you shouldn't), issue \begin{Spacing} only when really necessary.

\documentclass{memoir}
\usepackage[a4paper,margin=2cm]{geometry} % just to fit the example in one page
\usepackage{lipsum}

\newif\iftightlist
\newenvironment{mytightlist}[3]{%
  \iftightlist
    \def\finishspacing{}%
  \else
    \begin{Spacing}{1}\def\finishspacing{\end{Spacing}}%
  \fi
  \tightlisttrue
  \begin{list}{#1}{%
    \setlength{\topsep}{0pt}%
    \setlength{\parskip}{0pt}%
    \setlength{\parsep}{0pt}%
    \setlength{\itemsep}{0pt}%
    \setlength{\leftmargin}{#2}%
    \setlength{\labelwidth}{#3}%
    \setlength{\labelsep}{0.5em}%
  }%
}%
{\end{list}\finishspacing}


\begin{document}

\begin{Spacing}{1.5}

\lipsum*[2]

\begin{mytightlist}{\(\bullet\)}{1.5em}{0.5em}
\item \lipsum*[2]
  \begin{mytightlist}{\(\circ\)}{1.5em}{0.5em}
  \item \lipsum*[2]
  \item \lipsum*[2]
  \end{mytightlist}
\item \lipsum*[2]
\end{mytightlist}

\lipsum*[2]

\end{Spacing}

\end{document}