fold / unfold system of equations in long derivation
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}
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}