tcolorbox: It is possible to put breakable boxes behind each other (overlap)?

Question

Since breakable boxes in breakables boxes are not possible, as discussed in Breakable box in breakable box with tcolorbox and in the manual of tcolorbox.

So I thought of an alternative: is it possible to put breakable boxes behind each other (overlap)? The box in the background is a little bit larger than the box in the foreground, so it seems to be 'a box in a box', but in reality it are just breakable boxes outside each other. One would expect they break normally?

For example, from the the manual of tcolorbox. How can I simulate the 'the box in a box' by overlapping, so that one would expect they break normally?

Answer 1

This is just an idea.

As nested boxes are unbreakable a possible solution when we have only two nested boxes consist in defining the inner box as breakable and draw the outer one with some overlay options.

To simulate the inner box, we have to adjust margins with enlarge ... option and also define the corresponding width. Title, colframe and colback for outer box are fixed in overlay commands. Probably outer box geometry is not correct but I left to you to use better adjusted dimensions.

\documentclass{article}
\usepackage[most]{tcolorbox}
\usepackage{lipsum}

\newtcolorbox{myfakebreakablebox}[2][]{
    title=#2,
    enhanced, breakable, 
    enlarge top initially by=1cm,
    enlarge bottom finally by=5mm,
    enlarge left by=5mm,
    enlarge right by=5mm,
    width=\linewidth-10mm,
    overlay first={
        \draw[green!70!black, line width=.5mm, rounded corners]
            ([xshift=-5mm]frame.south west)|-([yshift=1cm]frame.north)-|
            ([xshift=5mm]frame.south east);
        \node[fill=green!70!black, minimum height=5mm, 
            minimum width=\linewidth, anchor=north] at ([yshift=1cm]frame.north) 
            (outertitle) {};
        \node[text=white, anchor=west] at ([xshift=3mm]outertitle.west) {Outer title};
        },
    overlay middle={
        \draw[green!70!black, line width=.5mm, rounded corners]
             ([xshift=-5mm]frame.north west)--([xshift=-5mm]frame.north west);
        \draw[green!70!black, line width=.5mm, rounded corners]
             ([xshift=-5mm]frame.north east)--([xshift=-5mm]frame.north east);
    },
    overlay last={
        \draw[green!70!black, line width=.5mm, rounded corners]
             ([xshift=-5mm]frame.north west)|-([yshift=-5mm]frame.south)
             -|([xshift=5mm]frame.north east);
    }
}

\begin{document}
\lipsum[1-3]
\begin{myfakebreakablebox}{this is the title}
\lipsum[1-3]
\end{myfakebreakablebox}
\lipsum[1]
\end{document}