Custom use case environment that can break across columns

Question

I created a custom environment to separate the semantics from design and formatting of use case tables. Everything works fine except the tabularx environment is unable to break across columns which creates nasty looking areas of white space when several (especially longer) use cases follow right after one another. Is there any way I can enable them to break across columns?

\documentclass[10pt]{scrartcl}

\usepackage[english]{babel}
\usepackage[margin=15mm]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[svgnames,table]{xcolor}

\usepackage{booktabs,tabularx,multicol,enumitem,blindtext}
\setlist{leftmargin=*,nosep}

\newcommand{\tabhead}[2][DarkBlue]{\textbf{\large\sffamily\color{#1} #2}}

\newcounter{usecase}
\newenvironment{usecase}[1]{
    \refstepcounter{usecase}
    \noindent\tabularx{\linewidth}{lX}
    \rowcolor{gray!20}\tabhead{Case~\theusecase} & \tabhead[DarkGreen]{#1}\\
    \toprule
    }
    {\bottomrule\endtabularx\medskip}

\newcommand{\actor}[1]{Actor & #1\\\midrule}

\newcommand{\precondition}[1]{Needs & #1\\\midrule}

\newcommand{\mainpath}[1]{
    Path & \begin{minipage}[t]{\linewidth}\blindenumerate[20]
    \end{minipage}\\}

\setlength{\footskip}{1cm}



\begin{document}

\title{The Title}
\author{Casimir}
\date{\today}
\maketitle

\begin{abstract}
    \blindtext
\end{abstract}

\begin{multicols}{2}

\section{First Section}

\blindtext

\subsection{First Subsection}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\end{multicols}

\end{document}

Update

New implementation with inspiration from David Carlisle.

\documentclass[10pt]{scrartcl}

\usepackage[english]{babel}
\usepackage[margin=15mm]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[svgnames,table]{xcolor}

\usepackage{multicol,enumitem,blindtext}
\setlist{leftmargin=*,nosep}

\newcommand{\uchead}[2][DarkBlue]{\textbf{\large\sffamily\color{#1} #2}}

\newcounter{usecase}
\newenvironment{usecase}[1]{
    \refstepcounter{usecase}
    \noindent\colorbox{gray!20}{%
       \makebox[\dimexpr\linewidth-2\fboxsep][l]{%
         \uchead{Case~\theusecase}~~\uchead[DarkGreen]{#1}}}\par
    \hrule}
    {\bigskip}

\newlength\lcol\setlength{\lcol}{1.2cm}
\newcommand{\ucentry}[1]{\noindent\strut\makebox[\lcol][l]{\textbf{#1}}}

\newcommand{\actor}[1]{%
    \ucentry{Actor} #1\hrule}

\newcommand{\precondition}[1]{%
    \ucentry{Needs} #1\hrule}

\newcommand{\mainpath}[1]{%
    \ucentry{Path}
    \vspace{-\dimexpr\parskip+\baselineskip}%
    \setlist[enumerate]{labelindent=\dimexpr\lcol+\fboxsep} #1\smallskip\hrule}

\setlength{\footskip}{1cm}



\begin{document}

\title{The Title}
\author{Casimir}
\date{\today}
\maketitle

\begin{abstract}
    \blindtext
\end{abstract}

\begin{multicols}{2}

\section{First Section}

\blindtext

\subsection{First Subsection}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{\blindenumerate[9]}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{\blindenumerate[9]}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{\blindenumerate[9]}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{\blindenumerate[9]}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{\blindenumerate[9]}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{\blindenumerate[9]}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{\blindenumerate[9]}
\end{usecase}

\end{multicols}

\end{document}

Answer 1

Things that look like lists are usually better set as lists than tables, then they break naturally.

You could fiddle a bit more with the spacing but:

\documentclass[10pt]{scrartcl}

\usepackage[english]{babel}
\usepackage[margin=15mm]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[svgnames,table]{xcolor}

\usepackage{booktabs,tabularx,multicol,enumitem,blindtext}
\setlist{leftmargin=*,nosep}

\newcommand{\tabhead}[2][DarkBlue]{\textbf{\large\sffamily\color{#1} #2}}

\newcounter{usecase}
\newenvironment{usecase}[1]{
    \refstepcounter{usecase}
    \colorbox{gray!20}{%
       \makebox[2cm][l]{\tabhead{Case~\theusecase}}%
       \hspace{6pt}%
       \makebox[\dimexpr\linewidth-3cm-6pt-2\fboxsep]{%
         \tabhead[DarkGreen]{#1}}}\par
    \hrule}
    {\hrule\medskip}

\newcommand{\actor}[1]{%
   \par
   \makebox[2cm][l]{\strut Actor}
       \hspace{6pt}%
   \mbox{#1}\par\hrule}

\newcommand{\precondition}[1]{%
   \par
   \makebox[2cm][l]{\strut Needs}
       \hspace{6pt}%
   \mbox{#1}\par\hrule}


\newcommand{\mainpath}[1]{%
   \par
   \makebox[2cm][l]{\strut Path}
    \par
   \nobreak
   \vspace{-\dimexpr\parskip+\baselineskip}%
    \setlist[enumerate]{leftmargin=2cm}%
   \blindenumerate[20]
  }

\setlength{\footskip}{1cm}



\begin{document}

\title{The Title}
\author{Casimir}
\date{\today}
\maketitle

\begin{abstract}
    \blindtext
\end{abstract}

\begin{multicols}{2}

\section{First Section}

\blindtext

\subsection{First Subsection}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\begin{usecase}{Some use case}
    \actor{Some actor}
    \precondition{User logged in}
    \mainpath{}
\end{usecase}

\end{multicols}

\end{document}