\setindent hack and alignment between parallel paragraphs with reledmac

Question

Based on David Carlisle's answer here, I've defined a macro that stores the length of the last line of the current paragraph. But when using it with reledmac and -par, it messes up the alignment parallel paragraphs. Does anyone understand why?

\documentclass{article}
\usepackage{reledmac}
\usepackage{reledpar}
\usepackage{lipsum}

\makeatletter
\def\setindent{%
    {\abovedisplayshortskip\z@\abovedisplayskip\z@
    \belowdisplayshortskip\z@\belowdisplayskip\z@
$$\global\dimen\@ne\predisplaysize
 \xdef\tmp{%
      \predisplaysize\the\predisplaysize
      \prevgraf\the\prevgraf\relax}%
$$\vskip\dimexpr-\parskip-\baselineskip\relax}\tmp
}
\dimen\@ne=\parindent

\begin{document}
\begin{pages}
\begin{Leftside}
\beginnumbering
\pstart
\lipsum*[1]
\pend

\pstart
\lipsum*[2]
\pend

\pstart
\lipsum*[3]
\pend

\pstart
\lipsum*[4]
\pend
\endnumbering
\end{Leftside}

\begin{Rightside}
\beginnumbering
\pstart
\lipsum*[5]\setindent
\pend

\pstart
\lipsum*[6]\setindent 
\pend

\pstart
\makeatletter
\noindent\hskip\dimen\@ne
\lipsum*[7]\setindent  
\pend

\pstart
\makeatletter
\noindent\hskip\dimen\@ne
\lipsum*[8]
\pend
\endnumbering
\end{Rightside}
\end{pages}
\Pages
\end{document}

Answer 1

Why your solution bugs

\pstart…\pend must contain only normal text. Not mathematical display, and not \vskip. Any entry like this perturbate the way reledmac/reledpar split lines, and so the alignment.

Principle of my solution

We will use a \setindent but OUTSIDE of the \pstart…\pend. To do that, we use the fact that in parallel typesetting, reledpar stores the \pstart…\pend content in numbered vbox. So we will copy the content of the box, and using some TeX' hacks on box mechanism, get the last line of the vbox. And then, using new code provide by David Carlisle, get the natural width of the lastline.

Other inhancement

As explained by chat, you should not use \maketaletter without a corresponding \makeatother. And if you use many time command with @ in the name, you should wrap it in an other command. I have created a \myindent command.

So, final code

\documentclass{article}
\usepackage{reledmac}
\usepackage{reledpar}
\usepackage{lipsum}

\makeatletter
% My new setindent
\def\setindent{%
\ifledRcol
  \setbox0=\expandafter\copy\csname l@dRcolrawbox\the\l@dnumpstartsR\endcsname%On rightside, copy the content of last \pstart…\pend pair
\else
  \setbox0=\expandafter\copy\csname l@dLcolrawbox\the\l@dnumpstartsL\endcsname%On leftside, copy the content of last \pstart…\pend pair
\fi
\expandafter\newdimen\csname l@\the\l@dnumpstartsR\endcsname%
\setbox0=\vbox{%
  \parindent=0pt%
  \setbox1=\vsplit0 to\baselineskip%Get only the last line in vbox0
  \unvbox0%Unvbox it
  \setbox0=\lastbox%Get the last line on the form of hbox
  \setbox0=\hbox{\unhbox0\unpenalty\unskip\unskip}
  \global\dimen\@ne=\wd0%
  }%We do it in a setbox in order to print nothing
}
% Avoid to use command with @ in the main text
\newcommand{\myindent}{\noindent\hskip\dimen\@ne}% I don't understand why we need such correction of 20pt !!! But it is not linked to reledmac,  as a test of your \setindent outside of any reledmac context can prove 
\makeatletter
\begin{document}
\begin{pages}

\begin{Leftside}
\beginnumbering
\pstart
\lipsum*[1]
\pend

\pstart
\lipsum*[2]
\pend

\pstart
\lipsum*[3]
\pend

\pstart
\lipsum*[4]
\pend
\endnumbering
\end{Leftside}

\begin{Rightside}
\beginnumbering
\pstart
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\pend
\setindent
\pstart
\myindent\lipsum*[6]%
\pend
\setindent 
\pstart
\myindent
\lipsum*[7]%
\pend
\setindent  
\pstart
\myindent\lipsum*[8]
\pend
\setindent  
\endnumbering
\end{Rightside}
\end{pages}
\Pages


\end{document}

A variant

Maybe instead of

\newcommand{\myindent}{\noindent\hskip\dimen\@ne}

Do

\newcommand{\myindent}{\noindent\ \hskip\dimen\@ne}

It will add a space, as required after a final dot.