Tighter-spaced bulleted list should have ordinary line spacing to paragraphs/lines above and below

Question

To get tighter line spacing (my document's main spacing is 1.5-spacing, and I'm using memoir), I normally use a self-defined "tight list" (tightlist in the code below). This works well. However I would like the list's spacing to the text above and below to be identical to the ordinary (here: 1.5) line spacing, that is I would like the list to behave like a vertical line-like atom with respect to the rest of the document.

In the following document

\documentclass{memoir}

\OnehalfSpacing

\newenvironment{mytightlist}[3]{%
  % noticeably tighter than the memoir class's tightlist facilities
  \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. HHHHHHHHHH

Text. HHHHHHHHHH

\begin{mytightlist}{\(\bullet\)}{1.5em}{0.5em}
\item Text. HHHHHHHHHH
\item Text. HHHHHHHHHH
\end{mytightlist}

Text. HHHHHHHHHH

Text. HHHHHHHHHH

\end{document}

the spacing within line pairs 2/3 and 4/5 is slightly less than the intended spacing, namely that found within line pairs 1/2 and 5/6. Why? How can one fix this?

Btw, I don't care about whether a list "belongs" to a paragraph around it: for me, such spacing should be uniform. ("LaTeX goes to a lot of effort to distinguish in-paragraph lists from end- or start-of-paragraph lists"; comment (quoted approximately) by David Carlisle)

Using the MySpacing environment from this answer to a related question about spacing around blocks instead of Spacing doesn't do the trick in this context.

Answer 1

You're using different baselineskips and you don't take care of this aspect.

\documentclass{memoir}

\newlength{\tightlisttopsep}
\AtBeginDocument{
  \setlength{\tightlisttopsep}{-\baselineskip}
  \addtolength{\tightlisttopsep}{\fontchardp\font`j}% a descender
  \addtolength{\tightlisttopsep}{.15pt} % some overshoot
  \OnehalfSpacing
  \addtolength{\tightlisttopsep}{.7\baselineskip}% height of strut
  % Show the baselines
  \edef\myrule{\noexpand\smash{\vrule depth\the\baselineskip\kern-.4pt}}
}

\newenvironment{mytightlist}[3]{%
  % noticeably tighter than the memoir class's tightlist facilities
  \begin{Spacing}{1}%
  \begin{list}{#1}{%
    \setlength{\topsep}{\tightlisttopsep}%
    \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}

1. HHHHHHHHH\myrule H

2. HHHHHHHHHH\myrule

\begin{mytightlist}{\(\bullet\)}{1.5em}{0.5em}
\item 3. HHHHHHHHHH
\item 4. \myrule HHHHHHHHHH
\end{mytightlist}

5. HHHHHHHHHH

6. HHHHHHHHHH

\end{document}

This is without hope if the mytightlist is used when a different font size is in force, because the settings are made just for the standard size.

The result is surely poor typography, in my opinion.

Answer 2

As to why it exists, I can't say. As to how to fix it (at least for your MWE), I add a symmetric \vspace of .049\baselineskip before and after the environment.

\documentclass{memoir}
\def\vertoffset{.049\baselineskip}

\OnehalfSpacing

\newenvironment{mytightlist}[3]{%
  % noticeably tighter than the memoir class's tightlist facilities
  \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}%
    \vspace{\vertoffset}%
  }%
}%
{\end{list}\vspace{\vertoffset}\end{Spacing}}


\begin{document}

Text. HHHHHHHHHH

Text. HHHHHHHHHH

\begin{mytightlist}{\(\bullet\)}{1.5em}{0.5em}
\item Text. HHHHHHHHHH
\item Text. HHHHHHHHHH
\end{mytightlist}

Text. HHHHHHHHHH

Text. HHHHHHHHHH

\end{document}

Answer 3

Reading your question I am not sure if you want also a 1.5 within the list environment. If this matter, then the fix of Steven is not complete since spacing between normal text lines items is15pt whereas between items is only 12pt.

My attempt to fix both problems is simply change two of the \setlenghts of your macro to:

\setlength{\topsep}{.7pt}% 
\setlength{\itemsep}{3pt}%

To see how good is this fix, an easy test is that \vspace{-15pt} after some line or item must produce a perfect overlapping with the next line or item. In the next image I used some \phantom spaces in overlapping lines and red color in the whole overlapped line-item to check it better. At 400% this is the output:

The source code to test yourself:

\documentclass{memoir}
\usepackage{xcolor}
\OnehalfSpacing

\newenvironment{mytightlist}[3]{%
  % noticeably tighter than the memoir class's tightlist facilities
  \begin{Spacing}{1}%
  \begin{list}{#1}{%
    \setlength{\topsep}{.7pt}% .1ex
    \setlength{\parskip}{0pt}%
    \setlength{\parsep}{0pt}%
    \setlength{\itemsep}{3pt}%
    \setlength{\leftmargin}{#2}%
    \setlength{\labelwidth}{#3}%
    \setlength{\labelsep}{0.5em}%
  }%
}%
{\end{list}\end{Spacing}}


\begin{document}


Text. {\color{red}HHHHHHHHHH (1-2 li\vspace{-15pt}}

Text. HHHHHHHHHH \phantom{(1-2 li}ne)

Text. HHHHHHHHHH (3 line)

Text. {\color{red}HHHHHHHHHH (4-1 line \vspace{-15pt}}

\begin{mytightlist}{\(\bullet\)}{1.5em}{.5em}
\item Text. HHHHHHHHHH  \phantom{(4-1 line}item) 
\item Text. HHHHHHHHHH (2 item) 
\item {\color{red}Text. HHHHHHHHHH (3-4 it}\vspace{-15pt}
\item Text. HHHHHHHHHH \phantom{(3-4 it}em) 
\item Text. HHHHHHHHHH (5 item)
\item {\color{red}Text. HHHHHHHHHH (6-1 item\vspace{-15pt}}
\end{mytightlist}

Text. HHHHHHHHHH \phantom{(6-5 item}line)

Text. HHHHHHHHHH (6 line)

Text. {\color{red}HHHHHHHHHH (7-8 li\vspace{-15pt}}

Text. HHHHHHHHHH \phantom{(7-8 li}ne)

\end{document}