How can I use titlesec to put a tcolorbox around a section title?

Question

I'd like to redefine \section to produce something like the following: (requiring package tcolorbox)

\begin{tcolorbox}
  1. \hspace*{\fill} Title of the Section \hspace*{\fill}
\end{tcolorbox}

I assumed I could use the titlesec package to do so. I tried doing the following:

\titleformat{\section}[display]% It's not clear to me why, but any other format except [display] breaks!
    {}{}{0em}%
    {\begin{tcolorbox}\thesection.\hspace*{\fill}}% 'before' code.
    [\hspace*{\fill}\end{tcolorbox}]% 'after' code.

The compiler accepts this, but not in the way that I expected it to. \section{Title of the Section} produces the following:

I know that titlesec does provide a boxed title option, but it is not in the format that I would like to use. In particular I would prefer for the label to also be in the box, and I would like the control over the box that tcolorbox provides me.

I thought perhaps I could define the contents of this section title box in the titlesec regime, then perhaps define \edef\oldsection\section and redefine \section as I like, but it's unclear to me how I might pass along the arguments that \section takes.

TL;DR I'd like to put \section titles inside a \tcolorbox as in the first example, but it doesn't work.

Answer 1

You may want to use load the tcolorbox packge with the explicit option, which allows you to use #1 as placeholder for the section title in the defintion of the \section command. Also, I think the block format is better suited here:

\documentclass{article}
\usepackage{tcolorbox}
\usepackage[explicit]{titlesec}

\titleformat{\section}[block]
    {}{}{0em}
    {\begin{tcolorbox}
        \thesection. \hspace*{\fill} #1 \hspace*{\fill}
    \end{tcolorbox}}

\begin{document}

\section{Title of the Section}

\end{document}

---

To make this work even for starred \section commands:

\documentclass{article}
\usepackage{tcolorbox}
\usepackage[explicit]{titlesec}

\titleformat{\section}[block]
    {}{}{0em}
    {\begin{tcolorbox}
        \thesection. \hspace*{\fill} #1 \hspace*{\fill}
    \end{tcolorbox}}

\titleformat{name=\section,numberless}[block]
    {}{}{0em}
    {\begin{tcolorbox}
        \hspace*{\fill} #1 \hspace*{\fill}
    \end{tcolorbox}}

\begin{document}

\section*{Title of the Section}

\section{Title of the Section}

\section*{Title of the Section}

\end{document}

---

As has been pointed out in the comments, this solution does not work nicely with longer titles. Since you are using the tcolorbox package already, you could enhance your box a bit to make it work for longer titles as well:

\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{skins}
\usepackage[explicit]{titlesec}

\NewTColorBox{tcsectionbox}{ o }{%
  enhanced,
  halign=center,
  left=\IfValueTF{#1}{10mm}{4mm},
  right=4mm,
  top=3mm,
  bottom=3mm,
  boxsep=0mm,
  overlay={
    \IfValueT{#1}{
        \node[anchor=north west, inner xsep=4mm, inner ysep=3mm] 
        at (interior.north west) {#1};
    }
  }
}

\titleformat{\section}[block]
    {}{}{0em}
    {\begin{tcsectionbox}[\thesection.]
        #1
    \end{tcsectionbox}}

\titleformat{name=\section,numberless}[block]
    {}{}{0em}
    {\begin{tcsectionbox}
        #1
    \end{tcsectionbox}}

\begin{document}

\section{A ridiculously long section title that is only used to show that even long titles are working fine}

\section*{A ridiculously long section title that is only used to show that even long titles are working fine}

\section{A short title}

\end{document}