[Tex/LaTex] 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.