Breakable nested tcolorbox

Question

I am currently using tcolorbox to create a rounded corner grey box within a rounded corner bordered box:

\documentclass{article}

\usepackage[breakable,skins]{tcolorbox}
\usepackage{lipsum}

\newtcolorbox{OuterBox}[1][]{%
    breakable,
    enhanced,
    colback=white,
    colframe=blue,
#1}%

\newtcolorbox{InnerBox}[1][]{%
    enforce breakable,
    enhanced,
    colback=gray,
    colframe=white,
#1}%

\begin{document}
    \begin{OuterBox}
        Just a line of text         
        \begin{InnerBox}
            \lipsum[1]          
            \lipsum[1]
            \lipsum[1]
            \lipsum[1]
        \end{InnerBox}
    \end{OuterBox}  
\end{document}

In the above example the nested tcolorbox is started on the next page instead of continuing straight after the contents of the outer box which is rather messy. In more complex documents it becomes very messy:

Preventing the inner box from being breakable fixes those graphical glitches and is the reason why the documentation cautions against its use:

In the unlikely case you really want to have the nested box to be breakable, use /tcb/enforce breakable for the nested box. But, a breakable box inside a breakable box will usually give a mess.

This is also a limitation of mdframed as it's documentation states:

A nested mdframed environment can’t be splitted.

Is there an alternative package that does support this use?

Answer 1

I propose a partial solution. The main shortcoming is its dependence on later changes made in the .tex file.

The idea is to introduce a command that ends the current inner box and starts a new one. If a breaking is needed, the command is inserted in the text and the desired split is achieved. Without forcing the breaking of the inner box (not needed). In the code below the command is \splitbox. Moreover, starting with the default values of the inner box, TikZ is used to construct sharp corners at the bottom of the current page and at the top of the next one for the split box.

\documentclass{article}
\usepackage{tikz}
\usepackage[breakable, skins]{tcolorbox}
\usepackage{lipsum}

\newtcolorbox{OuterBox}[1][]{%
  breakable,
  enhanced,
  colback=white,
  colframe=blue,
  #1
}

\newtcolorbox{InnerBox}[1][]{%
  enforce breakable,
  enhanced,
  colback=yellow,
  colframe=yellow,
  #1
}

\newcommand{\splitbox}{%
  \hfill
  \begin{tikzpicture}[remember picture, overlay]
    \draw[yellow, line width=1.5ex] (1.55em, -2.2ex) -- +(-1\textwidth-3.1em, 0);
  \end{tikzpicture}
  \end{InnerBox}
  \begin{InnerBox}
  \begin{tikzpicture}[remember picture, overlay]
    \draw[yellow, line width=1.5ex] (-1.55em, 3.2ex) -- +(1\textwidth+3.1em, 0);
  \end{tikzpicture}\hspace{-.7ex}
}

\title{Breaking inner boxes. Trial and error}
\date{\vspace{-8ex}}

\begin{document}
\maketitle

\section{The forced breaking is needed}
\lipsum[7]
\begin{OuterBox}
  Just a line of text.
  \lipsum[5]
  \begin{InnerBox}
    Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut pu-
    rus elit, vestibulum ut, placerat ac, adipiscing vitae, felis. Cur-
    abitur dictum gravida mauris. Nam arcu libero, nonummy eget,
    consectetuer id, vulputate a, magna. Donec vehicula augue eu
    neque. Pellentesque habitant morbi tristique senectus et netus et
    malesuada fames ac turpis egestas. Mauris ut leo. Cras viverra
    metus rhoncus sem. Nulla et lectus vestibulum urna fringilla ultri-
    ces. Phasellus eu tellus sit amet tortor gravida placerat. Integer
    sapien est, iaculis in, pretium quis, viverra ac, nunc. Praesent
    eget sem vel leo ultrices bibendum. Aenean faucibus. Morbi do-
    lor nulla, malesuada eu, pulvinar at, mollis ac, nulla. Curabitur
    auctor semper nulla. Donec varius orci eget risus. Duis nibh mi,
    congue eu, accumsan eleifend, sagittis quis, diam. Duis eget orci
    sit amet orci dignissim rutrum.
    \splitbox
    Nam dui ligula, fringilla a, euismod sodales, sollicitudin vel, wisi.
    Morbi auctor lorem non justo. Nam lacus libero, pretium at,
    lobortis vitae, ultricies et, tellus. Donec aliquet, tortor sed ac-
    cumsan bibendum, erat ligula aliquet magna, vitae ornare odio
    metus a mi. Morbi ac orci et nisl hendrerit mollis. Suspendisse
    ut massa. Cras nec ante. Pellentesque a nulla. Cum sociis na-
    toque penatibus et magnis dis parturient montes, nascetur ridicu-
    lus mus. Aliquam tincidunt urna. Nulla ullamcorper vestibulum
    turpis. Pellentesque cursus luctus mauris.
  \end{InnerBox}
  \lipsum[4]
\end{OuterBox}  
\lipsum[5-6]

\end{document}

Note that due to \lipsum[n] behavior with respect to the neighboring paragraphs, I inserted normal text in the inner box (as in a normal text file).