mdframed cannot reduce margin

Question

\documentclass[a4paper,10pt]{article}
\usepackage[framemethod=TikZ]{mdframed}
\usepackage{lipsum}
\usepackage{multicol}

\mdfdefinestyle{graybox}{
    splittopskip=0,%
    splitbottomskip=0,%
    frametitleaboveskip=0,
    frametitlebelowskip=0,
    skipabove=0,%
    skipbelow=0,%
    leftmargin=0,%
    rightmargin=0,%
    innertopmargin=2mm,%
    innerbottommargin=2mm,%
    roundcorner=2mm,%
    backgroundcolor=gray!20}


\begin{document}

\begin{mdframed}[style=graybox]
\begin{multicols}{2}
\lipsum[1]
\columnbreak
\lipsum[2]
\end{multicols}
\end{mdframed}

\end{document}

Result:

How do I remove that top margin inside the mdframe?

Already tried this but doesn't work:

\begin{document}
\begin{mdframed}[style=graybox]
\begin{multicols}{2}
\vskip -2\baselineskip
\lipsum[1]
\columnbreak
\vskip -2\baselineskip
\lipsum[2]
\end{multicols}
\end{mdframed}
\end{document}

Answer 1

You need to adjust the \multicolsep variable:

\documentclass[a4paper,10pt]{article}
\usepackage[framemethod=TikZ]{mdframed}
\usepackage{lipsum}
\usepackage{multicol}

\mdfdefinestyle{graybox}{
    splittopskip=0,%
    splitbottomskip=0,%
    frametitleaboveskip=0,
    frametitlebelowskip=0,
    skipabove=0,%
    skipbelow=0,%
    leftmargin=0,%
    rightmargin=0,%
    innertopmargin=2mm,%
    innerbottommargin=2mm,%
    roundcorner=2mm,%
    backgroundcolor=gray!20}


\begin{document}

\begin{mdframed}[style=graybox]
\setlength{\multicolsep}{2pt}
\begin{multicols}{2}
\lipsum[1]
\columnbreak
\lipsum[2]
\end{multicols}
\end{mdframed}

\end{document}

From the documentation of multicol:

As its first action, the multicols environment measures the current page to determine whether there is enough room for some portion of multicolumn output. This is controlled by the ⟨dimen⟩ variable \premulticols which can be changed by the user with ordinary LATEX commands. If the space is less than \premulticols, a new page is started. Otherwise, a \vskip of \multicolsep is added.

\multicolsep is kicking in as multicols is the first thing in the environment. By default \multicolsep is 12pt plus 4pt minus 3pt, which is the same order of magnitude as the baselineskip/blank line in a standard document.