parcolumns spacing problem when used with expex

Question

I get two problems when I use expex and parcolumns together.

1. A big gap appears in the non-glossed column.
2. A gap above the gloss appears, such that the two columns/paragraphs are no longer aligned.

Why do these problems arise, and how to overcome them?

EDIT #1: This question exhibits a similar problem, but the solution is not quite relevant to me.

MWE (XeLaTeX):

\documentclass{book}

\usepackage{parcolumns}

\usepackage{expex}
\usepackage{polyglossia}
\setdefaultlanguage{english}
\setotherlanguage{hebrew}
\newfontfamily\hebrewfont[Script=Hebrew]{FrankRuehl}
\newfontfamily\englishfont{Linux Libertine O}

\usepackage{lipsum}

\usepackage{changepage}
\strictpagecheck

\makeatletter
\def\pc@placeboxes{%
  \global\let\@tempa\relax%
  \hb@xt@\linewidth{%
    \vfuzz30ex %
    \vbadness\@M%
    \splittopskip\z@skip%
    \checkoddpage\ifoddpage
      \count@\z@%
      \loop\ifnum\count@<\pc@columncount%
        \advance\count@\@ne%
        \my@placeboxes@body%
      \repeat%
    \else
      \count@\pc@columncount%
      \loop\ifnum\count@>\z@%
        \my@placeboxes@body%
        \advance\count@\m@ne%
      \repeat%
    \fi
  }%
  \@tempa%
}

\def\my@placeboxes@body{%
  \expandafter\ifvoid\csname pc@column@\number\count@\endcsname%
    \hskip\csname pc@column@width@\number\count@\endcsname%
  \else%
    \expandafter\setbox\expandafter\@tempboxa%
      \expandafter\vsplit\csname pc@column@\number\count@\endcsname%
        to \dp\strutbox%
    \vbox{\unvbox\@tempboxa}%
  \fi%
  \expandafter\ifvoid\csname pc@column@\number\count@\endcsname%
  \else%
    \global\let\@tempa\pc@placeboxes%
  \fi%
  \ifnum\count@>\z@%
    \strut%
    \hfill%
    \ifpc@rulebetween%
      \vrule%
      \hfill%
    \fi%
  \fi%
}
\makeatother

\begin{document}

\begin{parcolumns}[%
  sloppy        = true,
  sloppyspaces  = true,
  nofirstindent = true,
  colwidths     = {1=0.67\textwidth, 2=0.29\textwidth}
]{2}

\colchunk[1]{\exdisplay\begingl

\gla Far far away behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. //

  \glb Far far away behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. //

  \glc Far far away behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. //

\endgl\xe
}

\colchunk[2]{\small%
  \lipsum[2]
}
\end{parcolumns}

\end{document}

Output:

EDIT #2: Simpler MWE (XeLaTeX):

\documentclass{book}

\usepackage{parcolumns}
\usepackage{expex}
\usepackage{lipsum}

\begin{document}

\begin{parcolumns}[%
  sloppy        = true,
  sloppyspaces  = true,
  nofirstindent = true,
  colwidths     = {1=0.67\textwidth, 2=0.29\textwidth}
]{2}

\colchunk[1]{\exdisplay\begingl

\gla Far far away behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. //

  \glb Far far away behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. //

  \glc Far far away behind the word mountains, far from the countries Vokalia and Consonantia, there live the blind texts. //

\endgl\xe
}

\colchunk[2]{\small%
  \lipsum[2]
}
\end{parcolumns}

\end{document}

Answer 1

The stuff between \begingl and \endgl is put into a box. Therefore, parcolumns and parallel are unable to do their job properly, because they only see a tall line.

The following hack unboxes the outer box. But the inner parts are still boxed.

\documentclass{book}

\usepackage{parcolumns}
\usepackage{expex}
\usepackage{lipsum}

\newsavebox\expexBox
\newenvironment{unvtop}{%
  \let\orgvtop\vtop
  \protected\def\vtop{%
    \let\vtop=\orgvtop
    \global\setbox\expexBox=\vtop
  }%
  \ignorespaces
}{%
  \unvbox\expexBox
  \ignorespacesafterend
}

\begin{document}

\begin{parcolumns}[%
  sloppy        = true,
  sloppyspaces  = true,
  nofirstindent = true,
  colwidths     = {1=0.67\textwidth, 2=0.29\textwidth}
]{2}

\colchunk[1]{%
  \begin{unvtop}
    \exdisplay\begingl

    \gla A Far far away behind the word mountains, far from the countries
      Vokalia and Consonantia, there live the blind texts.  //

    \glb B Far far away behind the word mountains, far from the countries
      Vokalia and Consonantia, there live the blind texts.  //

    \glc C Far far away behind the word mountains, far from the countries
      Vokalia and Consonantia, there live the blind texts.  //

  \endgl\xe
  \end{unvtop}
}

\colchunk[2]{\small%
  \lipsum[2]
}
\end{parcolumns}

\end{document}