Set description list `labelwidth` based on widest label (with mathtools loaded)

Question

I would like to set the description's width automatically to the widest. The solution at Automatically set description list `labelwidth` based on widest label? works for me, except if I have the mathtools package loaded.

The following MWE, which is the same as Gonzalo's but loads the mathtools package, is below:

\documentclass{article}
\usepackage{enumitem}
\usepackage{environ}
\usepackage{mathtools}

\newlength\widest
\makeatletter
\NewEnviron{ldescription}{%
  \vbox{%
    \global\setlength\widest{0pt}%
    \def\item[##1]{%
      \settowidth\@tempdima{\textbf{##1}}%
      \ifdim\@tempdima>\widest\global\setlength\widest{\@tempdima}\fi%
    }%
    \setbox0=\hbox{\BODY}%
  }
  \begin{description}[
    leftmargin=\dimexpr\widest+0.5em\relax,
    labelindent=0pt,
    labelwidth=\widest]
  \BODY
  \end{description}%
}
\makeatother

\begin{document}

\begin{ldescription}
\item[Short] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
\item[A really really long label] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
\end{ldescription}

\begin{ldescription}
\item[Short] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
\item[A medium label] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
\end{ldescription}

\end{document}

If I remove mathtools, it works. But I need mathtools.

Answer 1

I don't understand the cause of the problem, but for some reason when mathtools is loaded the change to \widest inside the \vbox{...} is only being made locally and it is not propagating globally (You can see this by adding some \typeout{...} commands in appropriate places). One fix to to use \newdimen rather than newlength to create \widest. With dimensions, rather than lengths, you get what you want.

Here is the modified working MWE:

\documentclass{article}
\usepackage{mathtools}
\usepackage{enumitem}
\usepackage{environ}

\newdimen\widest
\makeatletter
\NewEnviron{ldescription}{%
  \global\widest=0pt%
  \vbox{%
    \def\item[##1]{%
      \settowidth\@tempdima{\textbf{##1}}%
      \ifdim\@tempdima>\widest\global\widest=\@tempdima\fi%
    }%
    \setbox0=\hbox{\BODY}%
  }
  \begin{description}[
    leftmargin=\dimexpr\widest+0.5em\relax,
    labelindent=0pt,
    labelwidth=\widest]
  \BODY
  \end{description}%
}
\makeatother

\begin{document}

\begin{ldescription}
\item[Short] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
\item[A really really long label] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
\end{ldescription}

\begin{ldescription}
\item[Short] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
\item[A medium label] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
\end{ldescription}

\end{document}