Left bar in the margin of an environment with figures/tables

Question

I am writing a document with theorems and proofs. Since, some reader may not be interested by the proofs I want to highlight them so they can be skipped easily.

The best idea I came up with is to put a bar in the left margin of the proof (if you have any better ideas I am interested).

The problem is that some proofs may take more than one page and contain figures and/or tables. Currently I found a solution in a previous post which uses the package framed and the command \leftbar. It works for several pages but not for figures and tables.

\documentclass{article}

\usepackage{xcolor}
\usepackage{framed}

\renewenvironment{leftbar}[1][\hsize]
{%
    \def\FrameCommand
    {%
        {\color{black}\vrule width 0.5pt}%
        \hspace{0pt}%must no space.
        \fboxsep=\FrameSep\colorbox{white}%
    }%
    \MakeFramed{\hsize#1\advance\hsize-\width\FrameRestore}%
}
{\endMakeFramed}

 \newenvironment{proof}[1][Proof]{\leftbar\begin{trivlist}\item[\hskip\labelsep {\bfseries#1}]}{\end{trivlist}\endleftbar}

\def\fact{It is  practically a big lie  that LaTeX makes  you focus on
  the content without bothering about the layout.}

\begin{document}

\begin{proof}
  \fact
\end{proof} % Works

%\begin{proof}
%  \fact
%  \begin{figure}
%    \centering My figure
%    \caption{Test figure}
%  \end{figure}
%\end{proof} % Does not works

\end{document}

I don't really understand how environments work. Do you know if it's possible to make this work for proofs which include tables or figures?

(if you know why the bar starts one line before the proof and finish one line after I am also interested but it is not my main concern)

Thanks in advance.

Answer 1

Place the proof inside the figure environment, or else the "fact" may end up in a different location than the figure.

EDITED to allow for multi-figures and page breaks in proofs (by using the \captionof{figure}{} approach).

\documentclass{article}
\usepackage{caption}
\usepackage{xcolor}
\usepackage{framed}
\usepackage{lipsum}
\renewenvironment{leftbar}[1][\hsize]
{%
    \def\FrameCommand
    {%
        {\color{black}\vrule width 0.5pt}%
        \hspace{0pt}%must no space.
        \fboxsep=\FrameSep\colorbox{white}%
    }%
    \MakeFramed{\hsize#1\advance\hsize-\width\FrameRestore}%
}
{\endMakeFramed}

 \newenvironment{proof}[1][Proof]{\leftbar\begin{trivlist}\item[\hskip\labelsep {\bfseries#1}]}{\end{trivlist}\endleftbar}

\def\fact{It is  practically a big lie  that LaTeX makes  you focus on
  the content without bothering about the layout.}
\def\gappar{\vspace{1em}\par}
\begin{document}
\begin{proof}
  \fact
\end{proof} % Works

\begin{figure}[ht]
\begin{proof}
  \fact\gappar
   \centering\fbox{My figure}
    \caption{Test figure}
\end{proof} % Now it works
\end{figure}

\begin{proof}
  \fact\gappar
    {\centering\fbox{My figure 1}
    \captionof{figure}{Test figure 1}\gappar}
   \lipsum[4]\gappar
    {\centering\fbox{My figure 2}
    \captionof{figure}{Test figure 2}\gappar}
    QED
\end{proof} % Now it works
\end{document}

Answer 2

I have another solution where the word "Proof" is placed before the bar, and the bar can break across pages. It essentially uses the ntheorem and framed packages. Here is the code:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{lipsum}
\usepackage[svgnames, x11names]{xcolor}
%\usepackage[showframe,nomarginpar]{geometry}%
%\usepackage{fancybox}
\usepackage{xspace}
\usepackage{framed}
\usepackage[framed]{ntheorem}

\theoremstyle{nonumberplain}
\theorembodyfont{\upshape\small}
\theoremheaderfont{\color{IndianRed3}\sffamily\bfseries}%
\newlength{\FrameIndent}
\settowidth{\FrameIndent}{Proof\hspace{2\fboxsep}}%
\def\FrameCommand{%
\hspace*{\FrameIndent}\textcolor{IndianRed3}{\vrule width 0.6pt}\hspace{\fboxsep}
}%

\newframedtheorem{proof}{\llap{Proof\hspace{4\fboxsep}}\hspace{-\parindent}}%%{Proof}%

\begin{document}

\lipsum[1-3]

\begin{proof}
  \lipsum[4-6]
\end{proof} %

\end{document}