Sidenotes and marginfigures in tcolorbox theorems

Question

I am using the tufte-book class to write my notes and I would like to have tcolorbox theorems that can contain marginfigures and sidenotes (provided by the sidenotes package). Following I will post a MWE and the error I have in both situations:

\documentclass[nobib,notoc, a4paper]{tufte-book}

% For the example image
\usepackage{graphicx}

\usepackage[T1]{fontenc}
\usepackage[italian]{babel} % Italian support
\usepackage[utf8]{inputenc} % Accented letters

\usepackage[theorems]{tcolorbox}

\newtcbtheorem{Thm}{Teorema}%
{colback=green!5,colframe=green!35!black,fonttitle=\bfseries}{thm}

\begin{document}

\mainmatter

\begin{Thm}{Titolo}{esempio}
  Esempio di teorema.
  %\sidenote{Una nota a margine}
  \begin{marginfigure}
    \includegraphics{example-image-a}
    \caption{Figura di esempio}
    \label{fig:ex_a}
  \end{marginfigure}
\end{Thm}

\end{document}

When using the marginfigure environment I incur in

LaTeX Error: Not in outer par mode.

while when using a side note

l.29 LaTeX Error: Float(s) lost.
l.21 Missing number, treated as zero.

I had this problem not only with tcolorbox but also with ntheorem (that I am currently using but moving from because I would like different styles for theorems, definitions, examples etc.) I am basically forced to use marginfigures only outside of these environments making the positioning very tricky and the code quite messy.

I looked at this answer and this too but none of them actually solved my problem since I cannot put theorems inside marginfigures. From what I understand the problem is that both marginfigures and sidenotes are floats and so are tcolorbox' theorems and I can't create one from the other. I'm not grokking LaTeX internals well enough to develop a solution or to understand better.

Any insight for this would be greatly appreciated!

Answer 1

Here there are two possible solutions for marginfigures. I think OP wants something like the second, but ...

Sidenotes are another problem not solved here.

First solution uses a sidebyside box where upper part is the theorem contents and lower part a non floating figure which has been captioned with \captionof command.

Second solution uses a tcbraster with two columns. These columns have their own size, left one is the tcolorboxed theorem and the right one a non floating figure with a caption.

Some work on margins is needed but this is just a proof of concept.

By the way, neither sidebyside tcolorboxes nor rows from a tcbraster can be broken between pages. Therefore some pagination problems should be expected.

\documentclass[nobib,notoc, a4paper]{tufte-book}

% For the example image
% \usepackage{graphicx} %<- loaded by tcolorbox

\usepackage[T1]{fontenc}
\usepackage[italian]{babel} % Italian support
\usepackage[utf8]{inputenc} % Accented letters

\usepackage[most]{tcolorbox}

\usepackage{caption}
\usepackage{lipsum}

\tcbset{
    fullwidth/.style={
        enhanced,
        grow to right by=\marginparsep+\marginparwidth,
        sidebyside,
        righthand width=\marginparwidth,
        sidebyside gap=\marginparsep,
        segmentation hidden},
}

\newtcbtheorem{Thm}{Teorema}%
{colback=green!5,colframe=green!35!black,fonttitle=\bfseries}{thm}

\begin{document}

\mainmatter

\lipsum[2]

\begin{Thm}[fullwidth]{Titolo}{esempio}
\lipsum[2]
\tcblower
\centering
\includegraphics[width=\linewidth]{example-image-a}
\captionof{figure}{My first figure}
\end{Thm}

\begin{tcbraster}[
    raster width=\linewidth+\marginparsep+\marginparwidth,
    raster force size=false, 
    raster column skip=\marginparsep,]
\begin{Thm}[width=\textwidth]{Titolo}{esempio}
  Esempio di teorema.
\end{Thm}
\begin{tcolorbox}[blankest, width=\marginparwidth]
\includegraphics[width=\marginparwidth]{example-image-a}
    \captionof{figure}{Figura di esempio}
    \label{fig:ex_a}
  \end{tcolorbox}
\end{tcbraster}

\end{document}