tcolorbox: Potential bug with duplicate label for hyperref link

Question

I noticed that a 'tcolorbox' with auto counter including the section number causes a warning in the log file and the respective hyperref links point only to the first occurence.

MWE:

\documentclass{scrartcl}
\usepackage{tcolorbox}
\newtcolorbox[auto counter,number within=section]{mybox}[1][]{title=MyBox~\thetcbcounter,#1}
\usepackage{hyperref}

\begin{document}
Mybox~\ref{mybox1}; Mybox~\ref{mybox2}

\clearpage
\section{Section}
\begin{mybox}[label=mybox1]
The counter name is \texttt{\tcbcounter}.
\end{mybox}

\clearpage
\section{Section}
\begin{mybox}[label=mybox2]
The counter name is \texttt{\tcbcounter}.
\end{mybox}
\end{document}

The warning in the log file reads

warning  (pdf backend): ignoring duplicate destination with the name 'tcb@cnt@mybox.1'

So somehow the 'tcbcounter' is properly reset for the second section, but the internal counter 'tcb@cnt@mybox' does not contain the section number, so that the reference link to the second box points to the first.

Is this known or maybe a bug? Or can I, as the user, fix this easily? Thanks!

Answer 1

hyperref has a built-in mechanism for this. If for a counter the command \theH<counter> is defined, then it is used for internal anchor names instead of \the<counter>.

\documentclass{scrartcl}
\usepackage{tcolorbox}
\usepackage{hyperref}

\newtcolorbox[auto counter,number within=section]{mybox}[1][]{%
  title=MyBox~\thetcbcounter,
  #1
}
\providecommand{\theHtcbcounter}{\thesection.\arabic{tcbcounter}}

\begin{document}

Mybox~\ref{mybox1}; Mybox~\ref{mybox2}

\clearpage
\section{Section}
\begin{mybox}[label=mybox1]
The counter name is \texttt{\tcbcounter}.
\end{mybox}

\clearpage
\section{Section}
\begin{mybox}[label=mybox2]
The counter name is \texttt{\tcbcounter}.
\end{mybox}
\end{document}

This way the two links will have distinct anchor names and all is well.

If you want this to work with later loading of hyperref, you have to plunge in the internals:

\makeatletter
\providecommand\theHtcb@cnt@mybox{\thesection.\arabic{tcb@cnt@mybox}}
\makeatother

because tcolorbox associates that counter to the color box.

Answer 2

Based on @egreg's answer, it seems that one only needs to define new tcolorboxes after loading package hyperref. If both should be done from inside a custom package or a more complicated preamble structure, the \AtEndPreamble and \AtBeginDocument from the etoolbox package can be used.

Loading package auxhook early prevents an error message when using a koma-scrip class (see etoolbox + koma-script: Warning 'seems someone has broken package auxhook').

So a functional MWE would be:

\documentclass{scrartcl}
\usepackage{tcolorbox}
\usepackage{etoolbox, auxhook}

\AtEndPreamble{
    \usepackage{hyperref}
}

\AtBeginDocument{
    \newtcolorbox[auto counter,number within=section]{mybox}[1][]{%
        title=MyBox~\thetcbcounter,#1}
}

\begin{document}
Mybox~\ref{mybox1}; Mybox~\ref{mybox2}

\clearpage
\section{Section}
\begin{mybox}[label=mybox1]
The counter name is \texttt{\tcbcounter}.
\end{mybox}

\clearpage
\section{Section}
\begin{mybox}[label=mybox2]
The counter name is \texttt{\tcbcounter}.
\end{mybox}
\end{document}