Tcolorbox - footnotes at end of each page

Question

I am using tcolorbox to surround my text with a coloured box.

It seems that it cannot place footnotes at the end of each page. Instead, it places them at the end of the entire box. Demonstration below:

As you can see:

1. Footnote 1 appears at the end of the box (because I inserted it using the proper \footnote{} command.
2. Footnotes 2-4 are fake - I manually created them using superscripts in the text, and a combination of \hrule\, the \fakefill command, and the enumitem package.

How can I achieve something like the 'fake' footnotes I created with the \footnote command? I.e. placed at the bottom of the relevant page (not the end of the whole box).

In addition, I'd like the footnote text to be left-aligned with the footnote rule, similar to my fake footnotes, rather than indented in (like the actual footnote 1 at the end).

MWE for the picture above:

\documentclass[12pt]{article}
% look of the page
\usepackage[margin=0.8in, top = 1.3in, bottom = 1.3in, headheight = 0.6in]{geometry}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fancyhdr}
\renewcommand{\headrulewidth}{0pt}
\setlength{\footskip}{50pt}
\setlength{\footnotesep}{\baselineskip} % space between footnotes 
\setlength{\parindent}{0pt} % space at start of paragraph
\setlength{\parskip}{0.14in} % space between paragraphs
% math
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
% relevant to this question
\usepackage{enumitem} 
\usepackage[many]{tcolorbox}
\usepackage{xcolor}
\newcommand{\fakebreak}{\par\vspace{\textheight minus \textheight}\pagebreak}
\newcommand{\fakefill}{\par\vspace{\textheight minus \textheight}}
\begin{document}
\begin{tcolorbox}[%
parbox = false,
colback = white, colframe = black,
left = 0.5in, right = 0.5in, top = 0.4in, bottom = 0.4in,
height = 9.06in,
sharp corners,
boxrule = 1pt,
breakable,
]
\renewcommand{\thempfootnote}{\arabic{mpfootnote}}
First sentence.\footnote{Footnote 1.} \
Second sentence.$^2$
\fakefill
\par\noindent\rule{0.3\textwidth}{0.4pt}
{\footnotesize
\begin{enumerate}[leftmargin = 0in, rightmargin = 0in, topsep = 0pt]
\item[$^2$] Footnote 2.
\end{enumerate}
}
\newpage
Third sentence.$^3$\
Fourth sentence.$^4$
\fakefill
\par\noindent\rule{0.3\textwidth}{0.4pt}
{\footnotesize
\begin{enumerate}[leftmargin = 0in, rightmargin = 0in, topsep = 0pt]
\item[$^3$] Footnote 3.
\item[$^4$] Footnote 4.
\end{enumerate}
}
\end{tcolorbox}
\end{document}

Answer 1

By default, contents of tcolorbox environment is processed in a minipage env (set by capture=minipage). It is minipage that changes the behavior of \footnote.

In the following example, I emulate the normal behavior of \footnote by

• restoring footnote related counter (from mpfootnote to footnote), and
• postponing the insertion to \footins (normally done in \@footnotetext) till the end of first-layer tcolorbox.

Currently, all \footnotes used in a breakable tcolorbox are typeset as if they are used at the end of the last part.

Similar patches can be done on minipage, but the key-value system of tcolorbox makes the patch softer (less need to modify internal macros) and lighter (shorter codes).

• Update 2023-10-29
  • use collected footnotes before default /tcb/after ends the current paragraph, to prevent extra vertical space between adjacent tcolorboxes.
  • redirect footnotes in title and lower part as well

\documentclass{article}
\usepackage[margin=.5in, papersize={10cm, 17cm}]{geometry}
\usepackage[hooks]{tcolorbox}
\makeatletter
% restore footnote internals to those in normal page, not minipage
\def\tcb@restore@footnote{%
  \def@mpfn{footnote}%
  \def\thempfn{\arabic{footnote}}%
  \let@footnotetext\tcb@footnote@collect
}
% collect footnote text
\long\def\tcb@footnote@collect#1{%
  % expand @thefnmark before appending before app to \tcb@footnote@acc
  \expandafter\gappto\expandafter\tcb@footnote@acc\expandafter{%
    \expandafter\footnotetext\expandafter[@thefnmark]{#1}%
  }%
}
\def\tcb@footnote@use{%
  \tcb@footnote@acc
  \global\let\tcb@footnote@acc@empty
}
\global\let\tcb@footnote@acc@empty
\tcbset{
  % restore for every box
  every box/.style={
    before title pre=\tcb@restore@footnote, % just in case
    before upper pre=\tcb@restore@footnote,
    before lower pre=\tcb@restore@footnote,
  },
  % use for layer 1 boxes only
  every box on layer 1/.append style={
    % use \footnotetext befere the default /tcb/after ends the current paragraph
    after pre=\tcb@footnote@use
  }
}
\makeatother
\begin{document}
text\footnote{first}
\begin{tcolorbox}[title=Title\footnote{text}]
  content\footnote{inside tcolorbox}
  \tcblower
  footnote with optional argument\footnote[10]{inside tcolorbox}
\end{tcolorbox}
\begin{tcolorbox}
  another box immediately following
\end{tcolorbox}
text\footnote{middle}
\begin{tcolorbox}
  content\footnote{inside second tcolorbox}\par
  \begin{tcolorbox}
    content\footnote{layer \thetcblayer}
    \begin{tcolorbox}
      content\footnote{layer \thetcblayer}
    \end{tcolorbox}
    footnote with optional argument\footnote[20]{inside nested tcolorbox}
  \end{tcolorbox}
\end{tcolorbox}
text\footnote{last}
\end{document}

Answer 2

The footnotehyper package does the job easily. If the hyperlinks are unwanted, just don't use the hyperref package.

\documentclass{article}
\usepackage[papersize={10cm, 20cm}]{geometry}
\usepackage{hyperref}% optional
\usepackage{lipsum}
\usepackage{tcolorbox}
\usepackage{footnotehyper}
\makesavenoteenv{tcolorbox}
\begin{document}
text\footnote{first}
\begin{tcolorbox}
  content\footnote{inside tcolorbox}\par
  footnote with optional argument\footnote[10]{inside tcolorbox 2}
\end{tcolorbox}
text\footnote{third}
\begin{tcolorbox}
  content\footnote{inside second tcolorbox}\par

  footnote with optional argument\footnote[20]{inside second tcolorbox 2}
  \begin{tcolorbox}
    content\footnote{layer 2}
    \begin{tcolorbox}
      content\footnote{layer 3}
    \end{tcolorbox}
  \end{tcolorbox}
\end{tcolorbox}
text\footnote{seventh}
\end{document}