How to suppress vertical space between theorem heads and enumitem environments?

Question

How can I suppress vertical space between a theorem head and an enumitem environment? I would like the output to look like version 3 below, but I hope there is an automatical solution without ~ and \vspace.

\documentclass{article}
\usepackage{amsthm}
\usepackage{enumitem}
\newtheoremstyle{mythmstyle}%
{0.5em}% space above
{0.5em}% space below
{}% body font
{}% indent amount
{\sffamily\bfseries}% head font
{}% punctuation after head
{\newline}% space after head
{\thmname{#1}\ \thmnumber{#2}\ \thmnote{(#3)}}% head spec
\theoremstyle{mythmstyle}
\newtheorem{Theorem}{Theorem}
\begin{document}

%% version 1: vertical spacing correct
\begin{Theorem}
Dummy text
\begin{enumerate}[label=(\arabic*),leftmargin=*,align=left]
\item Item 1: More dummy text
\item Item 2: More dummy text
\end{enumerate}
\end{Theorem}

%% version 2: vertical spacing too large
\begin{Theorem}
~
\begin{enumerate}[label=(\arabic*),leftmargin=*,align=left]
\item Item 1: More dummy text
\item Item 2: More dummy text
\end{enumerate}
\end{Theorem}

%% version 3: desirable output, but ugly solution via \vspace
\begin{Theorem}
~\vspace{-1.4em}
\begin{enumerate}[label=(\arabic*),leftmargin=*,align=left]
\item Item 1: More dummy text
\item Item 2: More dummy text
\end{enumerate}
\end{Theorem}

%% version 4: does it work with enumitem alone? (no, topsep seems to be ignored)
\begin{Theorem}
~
\begin{enumerate}[label=(\arabic*),leftmargin=*,align=left, topsep=-1.4em]
\item Item 1: More dummy text
\item Item 2: More dummy text
\end{enumerate}
\end{Theorem}
\end{document}

Answer 1

Don't use ~ just use nothing (or a blank line) in the markup. You then have the problem that latex lists use a run-in style for the first item if following a run-in heading. This is by design (it goes to some trouble to make that work) If you don't like that style the thing to do is change that rather than put markup in the document. I don't know my way round that code as well as I used to, but this seems to work, it may need a bit more care to ensure that page breaks etc are suppressed in all the right places but...

\documentclass{article}
\usepackage{amsthm}
\usepackage{enumitem}
\newtheoremstyle{mythmstyle}%
{0.5em}% space above
{0.5em}% space below
{}% body font
{}% indent amount
{\sffamily\bfseries}% head font
{}% punctuation after head
{\newline}% space after head
{\thmname{#1}\ \thmnumber{#2}\ \thmnote{(#3)}}% head spec
\theoremstyle{mythmstyle}
\newtheorem{Theorem}{Theorem}


\makeatletter
\def\enumfix{%
\if@inlabel
 \noindent \par\nobreak\vskip-\parskip\vskip-\baselineskip\hrule\@height\z@
\fi}

\let\oldenumerate\enumerate
\def\enumerate{\enumfix\oldenumerate}


\begin{document}

%% version 1: vertical spacing correct
\begin{Theorem}
Dummy text
\begin{enumerate}[label=(\arabic*),leftmargin=*,align=left]
\item Item 1: More dummy text
\item Item 2: More dummy text
\end{enumerate}
\end{Theorem}


%% version 2a: vertical spacing too large
\begin{Theorem}

\begin{enumerate}[label=(\arabic*),leftmargin=*,align=left]
\item Item 1: More dummy text
\item Item 2: More dummy text
\end{enumerate}
\end{Theorem}
\end{document}

Answer 2

your approach may be ugly, and it really should use a vertical unit like ex instead of em, but if it gives you the result you want, why not just embed it into a \newenvironment?

for example,

\newenvironment{thmenumerate}{%
\leavevmode\vspace{-1.4em}
\begin{enumerate}[label=(\arabic*),leftmargin=*,align=left]
}{%
\end{enumerate}
}

and use it like this:

\begin{Theorem}
\begin{thmenumerate}
\item Item 1: More dummy text
\item Item 2: More dummy text
\end{thmenumerate}
\end{Theorem}