Extra space above `amsmath` environment inside `mdframed`

Question

When I use this code some space above appears.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mdframed}

\begin{document}

\begin{mdframed}    
\begin{gather*}
    \lambda^2-2\lambda = 0\\
    \lambda_1 = 0, \enspace \lambda_2 = 2
\end{gather*}
\end{mdframed}

\end{document}

I tried to implement this answer by egreg and it works, however I am getting these errors.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mdframed}
\usepackage{etoolbox}
\makeatletter
\pretocmd\start@gather{%
    \if@mdframed\kern-\topskip\kern-\abovedisplayskip\fi
}{}{}
\makeatother

\begin{document}

\begin{mdframed}    
\begin{gather*}
    \lambda^2-2\lambda = 0\\
    \lambda_1 = 0, \enspace \lambda_2 = 2
\end{gather*}
\end{mdframed}

\end{document}

---

Also I would like to know how to make it work for all the amsmath envinronments. Now only one with \start@gather is affected.

Answer 1

As @UlrikeFischer pointed in the question comment, there is no boolean \if@mdframed in mdframed package. The following workaround

• uses the value of TeX count \mdf@envdepth to determine if the code is executed inside mdframed environtment, and
• patches each of \start@gather, \start@align and \start@multline.

Hope that would work for every amsmath multiline displayed equation environment.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mdframed}
\usepackage{etoolbox}

\makeatletter
\pretocmd\start@gather{%
  \ifnum\mdf@envdepth>0
    \kern\glueexpr-\topskip-\abovedisplayskip\relax
  \fi
}{}{\fail}
\pretocmd\start@align{%
  \ifnum\mdf@envdepth>0
    \kern\glueexpr-\topskip-\abovedisplayskip\relax
  \fi
}{}{\fail}
\pretocmd\start@multline{%
  \ifnum\mdf@envdepth>0
    \kern\glueexpr-\topskip-\abovedisplayskip\relax
  \fi
}{}{\fail}
\makeatother

\begin{document}

gather:
\begin{mdframed}    
\begin{gather*}
    \lambda^2-2\lambda = 0\\
    \lambda_1 = 0, \enspace \lambda_2 = 2
\end{gather*}
\end{mdframed}

align:
\begin{mdframed}    
\begin{align*}
    \lambda^2-2\lambda = 0\\
    \lambda_1 = 0, \enspace \lambda_2 = 2
\end{align*}
\end{mdframed}

multline:
\begin{mdframed}    
\begin{multline*}
    \lambda^2-2\lambda = 0\\
    \lambda_1 = 0, \enspace \lambda_2 = 2
\end{multline*}
\end{mdframed}

\end{document}

---

A looping version in case you prefer a more compact patching code:

\makeatletter
\@for\@tempa:=gather,align,multline\do{%
  \expandafter\pretocmd\csname start@\@tempa\endcsname{%
    \ifnum\mdf@envdepth>0
      \kern\glueexpr-\topskip-\abovedisplayskip\relax
    \fi
  }{}{\fail}
}
\makeatother

Here \@for is a LaTeX2e internal macro which is documented in, for example macros2e documentation.