Controlling indent and vertical space at top of a tcolorbox

Question

I would like to set up a tcolorbox where the first paragraph is not indented, but subsequent paragraphs are indented.

I can accomplish this by passing an option like this to the box:

before upper=\setlength{\parindent}{1em}\noindent

The problem with this is that the \noindent causes TeX to enter horizontal mode. So if I start the box with something that should be in vertical mode (e.g., \section), extra vertical is inserted.

Is there a way I can automatically get the output I want without having to create two different kinds of boxes?

MWE

\documentclass{article}

\usepackage{tcolorbox}

\tcbset{mybox/.style={colback=white, colframe=blue, left=2mm, right=2mm,
  fonttitle=\bfseries}, fontupper=\small,
  before upper=\setlength{\parindent}{1em}\noindent}

\newtcolorbox{mybox}[1][]{mybox,#1}

\begin{document}

\begin{mybox}
  The first paragraph should not have an indent.

  Subsequent paragraphs should be indented.
\end{mybox}

\begin{mybox}
  \section{Should not have gap above this heading}
\end{mybox}

\begin{mybox}[before upper=\setlength{\parindent}{1em}]
  \section{Previous box should look like this}
\end{mybox}

\end{document}

Answer 1

You can use a simplified version of \@afterheading:

\documentclass{article}

\usepackage{tcolorbox}

\tcbset{mybox/.style={colback=white, colframe=blue, left=2mm, right=2mm,
  fonttitle=\bfseries}, fontupper=\small,
  before upper=\setlength{\parindent}{1em}\everypar{{\setbox0\lastbox}\everypar{}},
}

\newtcolorbox{mybox}[1][]{mybox,#1}

\begin{document}

\begin{mybox}
  The first paragraph should not have an indent.

  Subsequent paragraphs should be indented.
\end{mybox}

\begin{mybox}
  \section{Should not have gap above this heading}
\end{mybox}

\begin{mybox}[before upper=\setlength{\parindent}{1em}]
  \section{Previous box should look like this}
\end{mybox}

\end{document}

The tokens in \everypar are inserted after the paragraph has started, so just after the indentation box; with {\setbox0\lastbox} the indentation box is removed and put in box 0, which is then reset to what it was because the assignment happens in a group. Then \everypar is reset to empty.