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?

mdframed perhaps. Please note that pagebreaking and nesting is a very difficult issue. — , May 17 '16 at 16:11
Did you try the option to see if you get the usual mess or what you want? — cfr, May 17 '16 at 22:51
Note that the option does work (messily) if the inner box exceeds the available height within the box for a page. It just prefers breaking before the inner box to breaking within it. So if you put some of the stuff in one inner box and everything else in another, it does break. Messy, a bit, but it does kind of work. — cfr, May 17 '16 at 23:05
Will take a look at mdframed and have updated my question to show how the inner box is forced to start a new page if the inner box needs to break which is messy and in larger documents causes graphics glitches. — A Dark Divided Gem, May 17 '16 at 23:37
@ChristianHupfer Unfortunately mdframed doesn't support nesting. It must be a very difficult issue if its not fully supported by mature packages like mdframed or tcolorbox — A Dark Divided Gem, May 17 '16 at 23:59
@ADarkDividedGem: Can you elaborate on what you mean by mdframed does not support nesting? I use that all the time. See for instance How to get more than 3 levels of nesting in mdframed environments? — Peter Grill, May 18 '16 at 00:16
@ChristianHupfer Sorry I meant to say nested mdframed environments can't be broken/split over a page. I have updated my question to show the documentation lists that as known limitation. — A Dark Divided Gem, May 18 '16 at 00:28
^^ @PeterGrill I think the postman delivered your mail to the wrong address. — cfr, May 18 '16 at 01:36
Maybe try playing around with the break at option. I needed a breakable inside a breakable and it wound up splitting the inner breakable box over 10+ pages, only displaying perhaps 2 or so lines per pages. Setting break at=15cm seemed to fix this. I then had to play around and tweak the 15cm manually for the best appearance. — Jonathan Gleason, Jul 06 '16 at 03:51

score 1 · Accepted Answer · answered Sep 20 '20 at 08:46

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).

Thanks that was helpful, how does your solution compare to my current workaround https://tex.stackexchange.com/a/355194/43952 — A Dark Divided Gem, Sep 21 '20 at 09:52
The way I understand the problem is the following: having an inner box that must split on two pages, how can we handle it with minimal modification of the .tex file. My point of view is to have the splitting dealt with in the background through one command. This implies, in case some graphical properties are needed (colored frames and/or rounded boxes), to modify the graphical behavior by hand outside the boxes declaration. If not, the call of the inner box has to be modified each time when splitting occurs---the choice is among normal box, split bottom box, and spit upper box. — Daniel N, Sep 22 '20 at 07:41

Breakable nested tcolorbox

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