fold / unfold system of equations in long derivation

Question

I am showing a long derivation (simplified example here)

\documentclass{article}

\begin{document}

\begin{eqnarray*}
0 &=& 0 \\
&\downarrow& \mbox{using eq.}~\ref{eq0} \\
&=& 1 \\
&\downarrow& \mbox{using eq.}~\ref{eq1} \\
&=& 2 \\
&\downarrow& \mbox{using eq.}~\ref{eq2} \\
&=& 3
\end{eqnarray*}

\begin{equation}
0
\label{eq0}
\end{equation}

\begin{equation}
1
\label{eq1}
\end{equation}

\begin{equation}
2
\label{eq2}
\end{equation}

\end{document}

I would like to have only a folded representation showing only the line "0=0" and the final "=3" in the produced PDF. The desired behaviour would be that there is a button between these 2 final lines that, when hovered or pressed, would trigger a pop up showing the entire derivation.

The objective is that the reader can quickly see the start and end without having to see all the details in between at first view. Of course, I could put the derivation in an appendix but navigation would become difficult (especially considering that the intermediate equations themselves refer to other equations).

This question is very much related to: Collapsable/expandable text in Latex

But I couldn't find a way to adapt the code with the aligned equations. The picture below gives an example of the desired behaviour.

Answer 1

Building on J. Kormylo's example without vertical gap and making use of the

↗\tootip****{...}{...}[<x-off>][<y-off>] macro (Evince, A-Reader):

\documentclass{article}
\usepackage{mathtools}

\usepackage{hyperref}
%patch hyperref to make PDF Annotations PDF-Layer-(OCG)-aware
\makeatletter
  \let\Hy@setpdfborderOrig\Hy@setpdfborder
  \def\Hy@setpdfborder{\ocgbase@insert@oc\Hy@setpdfborderOrig}%
\makeatother
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% tooltips with LaTeX v. 2018/01/15
%
% \tooltip[*[*[*[*]]]]
%            [<link colour>]{<link text>}
%            [<tip box colour>]{<tip text>}
%            [<x-offset>,<y-offset>]
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%   \tooltip     --> draggable tip, visible on mouse-over, hidden on mouse-out
%
%   \tooltip*    --> draggable tip, toggle visiblity on mouse-over
%
%   \tooltip**   --> NON-draggable tip, visible on mouse-over, hidden on mouse-out
%
%   \tooltip***  --> NON-draggable tip, toggle visiblity on mouse-over
%
%   \tooltip**** --> NON-draggable tip, toggle visiblity on mouse-click (Evince!)
%
% Default link colour can be set with
%
%   \usepackage[linkcolor=<colour>]{hyperref}
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage{xparse,ocgbase}
\usepackage{xcolor,calc}
\usepackage{tikzpagenodes,linegoal}
\usetikzlibrary{calc}

\ExplSyntaxOn
\let\tpPdfLink\pbs_pdflink:nn
\let\tpPdfAnnot\pbs_pdfannot:nnnn\let\tpPdfLastAnn\pbs_pdflastann:
\let\tpAppendToFields\pbs_appendtofields:n
\def\tpPdfXform{\pbs_pdfxform:nnnnn{1}{1}{}{}}
\let\tpPdfLastXform\pbs_pdflastxform:
\let\cListSet\clist_set:Nn\let\cListItem\clist_item:Nn
\ExplSyntaxOff

\makeatletter
\NewDocumentCommand{\tooltip}{%
  ssssO{\ifdefined\@linkcolor\@linkcolor\else blue\fi}mO{yellow!20}mO{0pt,0pt}%
}{{%
  \leavevmode%
  \IfBooleanT{#2}{%
    %for variants with two and more stars, put tip box on a PDF Layer (OCG)
    \ocgbase@new@ocg{tipOCG.\thetcnt}{%
      /Print<</PrintState/OFF>>/Export<</ExportState/OFF>>%
    }{false}%
    \xdef\tpTipOcg{\ocgbase@last@ocg}%
    %prevent simultaneous visibility of multiple non-draggable tooltips
    \ocgbase@add@ocg@to@radiobtn@grp{tool@tips}{\ocgbase@last@ocg}%
  }%
  \tpPdfLink{%
    \IfBooleanTF{#4}{%
      /Subtype/Link/Border[0 0 0]/A <</S/SetOCGState/State [/Toggle \tpTipOcg]>>
    }{%
      /Subtype/Screen%
      /AA<<%
        \IfBooleanTF{#3}{%
          /E<</S/SetOCGState/State [/Toggle \tpTipOcg]>>%
        }{%
          \IfBooleanTF{#2}{%
            /E<</S/SetOCGState/State [/ON \tpTipOcg]>>%
            /X<</S/SetOCGState/State [/OFF \tpTipOcg]>>%
          }{
            \IfBooleanTF{#1}{%
              /E<</S/JavaScript/JS(%
                var fd=this.getField('tip.\thetcnt');%
                if(typeof(click\thetcnt)=='undefined'){%
                  var click\thetcnt=false;%
                  var fdor\thetcnt=fd.rect;var dragging\thetcnt=false;%
                }%
                if(fd.display==display.hidden){%
                  fd.delay=true;fd.display=display.visible;fd.delay=false;%
                }else{%
                  if(!click\thetcnt&&!dragging\thetcnt){fd.display=display.hidden;}%
                  if(!dragging\thetcnt){click\thetcnt=false;}%
                }%
                this.dirty=false;%
              )>>%
            }{%
              /E<</S/JavaScript/JS(%
                var fd=this.getField('tip.\thetcnt');%
                if(typeof(click\thetcnt)=='undefined'){%
                  var click\thetcnt=false;%
                  var fdor\thetcnt=fd.rect;var dragging\thetcnt=false;%
                }%
                if(fd.display==display.hidden){%
                  fd.delay=true;fd.display=display.visible;fd.delay=false;%
                }%
               this.dirty=false;%
              )>>%
              /X<</S/JavaScript/JS(%
                if(!click\thetcnt&&!dragging\thetcnt){fd.display=display.hidden;}%
                if(!dragging\thetcnt){click\thetcnt=false;}%
                this.dirty=false;%
              )>>%
            }%
            /U<</S/JavaScript/JS(click\thetcnt=true;this.dirty=false;)>>%
            /PC<</S/JavaScript/JS (%
              var fd=this.getField('tip.\thetcnt');%
              try{fd.rect=fdor\thetcnt;}catch(e){}%
              fd.display=display.hidden;this.dirty=false;%
            )>>%
            /PO<</S/JavaScript/JS(this.dirty=false;)>>%
          }%
        }%
      >>%
    }%
  }{{\color{#5}#6}}%
  \sbox\tiptext{%
    \IfBooleanT{#2}{%
      \ocgbase@oc@bdc{\tpTipOcg}\ocgbase@open@stack@push{\tpTipOcg}}%
    \fcolorbox{black}{#7}{#8}%
    \IfBooleanT{#2}{\ocgbase@oc@emc\ocgbase@open@stack@pop\tpNull}%
  }%
  \cListSet\tpOffsets{#9}%
  \edef\twd{\the\wd\tiptext}%
  \edef\tht{\the\ht\tiptext}%
  \edef\tdp{\the\dp\tiptext}%
  \tipshift=0pt%
  \IfBooleanTF{#2}{%
    %OCG-based (that is, all non-draggable) boxes should not extend beyond the
    %current column as they may get overlaid by text in the neighbouring column
    \setlength\whatsleft{\linegoal}%
  }{%
    \measureremainder{\whatsleft}%
  }%
  \ifdim\whatsleft<\dimexpr\twd+\cListItem\tpOffsets{1}\relax%
    \setlength\tipshift{\whatsleft-\twd-\cListItem\tpOffsets{1}}\fi%
  \IfBooleanF{#2}{\tpPdfXform{\tiptext}}%
  \raisebox{\heightof{#6}+\tdp+\cListItem\tpOffsets{2}}[0pt][0pt]{%
    \makebox[0pt][l]{\hspace{\dimexpr\tipshift+\cListItem\tpOffsets{1}\relax}%
    \IfBooleanTF{#2}{\usebox{\tiptext}}{%
      \tpPdfAnnot{\twd}{\tht}{\tdp}{%
        /Subtype/Widget/FT/Btn/T (tip.\thetcnt)%
        /AP<</N \tpPdfLastXform>>%
        /MK<</TP 1/I \tpPdfLastXform/IF<</S/A/FB true/A [0.0 0.0]>>>>%
        /Ff 65536/F 3%
        /AA <<%
          /U <<%
            /S/JavaScript/JS(%
              var fd=event.target;%
              var mX=this.mouseX;var mY=this.mouseY;%
              var drag=function(){%
                var nX=this.mouseX;var nY=this.mouseY;%
                var dX=nX-mX;var dY=nY-mY;%
                var fdr=fd.rect;%
                fdr[0]+=dX;fdr[1]+=dY;fdr[2]+=dX;fdr[3]+=dY;%
                fd.rect=fdr;mX=nX;mY=nY;%
              };%
              if(!dragging\thetcnt){%
                dragging\thetcnt=true;Int=app.setInterval("drag()",1);%
              }%
              else{app.clearInterval(Int);dragging\thetcnt=false;}%
              this.dirty=false;%
            )%
          >>%
        >>%
      }%
      \tpAppendToFields{\tpPdfLastAnn}%
    }%
  }}%
  \stepcounter{tcnt}%
}}
\makeatother
\newsavebox\tiptext\newcounter{tcnt}
\newlength{\whatsleft}\newlength{\tipshift}
\newcommand{\measureremainder}[1]{%
  \begin{tikzpicture}[overlay,remember picture]
    \path let \p0 = (0,0), \p1 = (current page.east) in
      [/utils/exec={\pgfmathsetlength#1{\x1-\x0}\global#1=#1}];
  \end{tikzpicture}%
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newcommand{\centerwithin}[2]{% #1=small symbol, #2=wide symbol
  {\mathmakebox[\widthof{\ensuremath{{}#2{}}}][c]{{#1}}}}

\begin{document}

\newcommand\tempbox{%
\begin{minipage}{0.18\textwidth}% almost impassible to pre-measure
\abovedisplayskip=0pt
\belowdisplayskip=0pt
\begin{align*}
&\centerwithin{\downarrow}{=} \text{using eq.~\eqref{eq0}} \\
&= 1 \\
&\centerwithin{\downarrow}{=} \mbox{using eq.~\eqref{eq1}} \\
&= 2 \\
&\centerwithin{\downarrow}{=} \mbox{using eq.}~\eqref{eq2}
\end{align*}
\end{minipage}}

\begin{align*}
0 &= 0\\
  &\makebox[0pt][r]{%
     \tooltip****{\fbox{\bfseries +}}{\tempbox}[-0.4em,-4ex]%
   }\\
  & = 3
\end{align*}

\begin{equation}
0
\label{eq0}
\end{equation}

\begin{equation}
1
\label{eq1}
\end{equation}

\begin{equation}
2
\label{eq2}
\end{equation}

\end{document}

Answer 2

I wasn't sure what to do with the equations (1) through (3), so I just left them as is. I used \tempbox to separate the long derivation from the OCG code, and \switchbox to align the derivation to the equation.

Note the use of flalign to left justify the derivation. Since equations expand to fill the entire column, it would be difficult to measure the width needed.

\documentclass{article}
\usepackage{mathtools}
\usepackage{ocgx}

\newsavebox{\tempbox}% simplify source
\newsavebox{\switchbox}% measure width
\savebox{\switchbox}{\fcolorbox{blue}{red}{\bfseries +}}

\newcommand{\centerwithin}[2]{% #1=small symbol, #2=wide symbol
  {\mathmakebox[\widthof{\ensuremath{{}#2{}}}][c]{{#1}}}}

\begin{document}

\savebox{\tempbox}{\begin{minipage}{0.5\textwidth}% almost impassible to pre-measure
\abovedisplayskip=0pt
\belowdisplayskip=0pt
\begin{flalign*}
&\centerwithin{\downarrow}{=} \text{using eq.~\ref{eq0}} &\\
&= 1 \\
&\centerwithin{\downarrow}{=} \mbox{using eq.~\ref{eq1}} &\\
&= 2 \\
&\centerwithin{\downarrow}{=} \mbox{using eq.}~\ref{eq2}
\end{flalign*}
\end{minipage}}

\begin{align*}
0 &= 0\\
&\mbox{\hspace*{-\wd\switchbox}\switchocg{ocg1}{\usebox{\switchbox}}\begin{ocg}{OCG 1}{ocg1}{0}
  \usebox{\tempbox}%
  \end{ocg}}\\
& = 3
\end{align*}

\begin{equation}
0
\label{eq0}
\end{equation}

\begin{equation}
1
\label{eq1}
\end{equation}

\begin{equation}
2
\label{eq2}
\end{equation}

\end{document}

---

Here is a variant which overlaps the text. Notice that only the text that comes after the OCG is visible.

\documentclass{article}
\usepackage{mathtools}
\usepackage{ocgx}

\newsavebox{\tempbox}% simplify source
\newsavebox{\switchbox}% measure width
\savebox{\switchbox}{\fcolorbox{blue}{red}{\bfseries +}}

\newcommand{\centerwithin}[2]{% #1=small symbol, #2=wide symbol
  {\mathmakebox[\widthof{\ensuremath{{}#2{}}}][c]{{#1}}}}

\begin{document}

\savebox{\tempbox}{\begin{minipage}{\dimexpr\textwidth-\wd\switchbox-0.666ex}%

\abovedisplayskip=0pt
\belowdisplayskip=0pt
\begin{align*}
0 &= 0 \\
&\centerwithin{\downarrow}{=} \text{using eq.~\ref{eq0}} \\
&= 1 \\
&\centerwithin{\downarrow}{=} \mbox{using eq.~\ref{eq1}} \\
&= 2 \\
&\centerwithin{\downarrow}{=} \mbox{using eq.}~\ref{eq2} \\
&= 3
\end{align*}
\end{minipage}}

\begin{flalign*}
&&0 &= 0 &\\
\rlap{\switchocg{ocg1}{\usebox{\switchbox}}\begin{ocg}{OCG 1}{ocg1}{0}
  \smash{\fcolorbox{black}{yellow}{\usebox{\tempbox}}}%
  \end{ocg}} &&&&\\
&&& = 3
\end{flalign*}

\begin{equation}
0
\label{eq0}
\end{equation}

\begin{equation}
1
\label{eq1}
\end{equation}

\begin{equation}
2
\label{eq2}
\end{equation}

\end{document}