Vertical and horizontal alignment of cells in a tabular with forests in them

Question

Here is a MWE in which I create a table with one row of plain text, another row with tikz node, and yet another one with forest cells. I try to use the [t] option to the table environment to top align the cells and column specifier c to center the cells vertically.

\documentclass{standalone}
\usepackage{forest}
\begin{document}
\forestset{c/.style={circle,draw},t/.style={},}
\def\bb{ \draw[fill=blue,opacity=0.2] (current bounding box.south west) rectangle (current bounding box.north east);}
\def\xx#1#2{
  #1
  \begin{tikzpicture}
    \node [fill=green,opacity=0.2,draw,minimum width=#1cm, minimum height=#2cm] {#1#2};
  \end{tikzpicture}
  #2
}
\begin{tabular}[t]{|c|c|c|c|c|c|}
  $(ab.cd)$                                                           &
  $(abcd.\bot)$                                                       &
  $(ab.(c.d))$                                                        &
  $((a.b).(c.d))$                                                     &
  $(a.(b.(c.d)))$                                                     &
  $(((a.b).c).d)$                                                       \\
  \hline
  \xx23                                                               &
  \xx34                                                               &
  \xx24                                                               &
  \xx14                                                               &
  \xx41                                                               &
  \xx43                                                                 \\
  \hline
  \scriptsize
  \Forest{for tree [{},c[$ab$,t][$cd$,t]]\bb}                         &
  \scriptsize
  \Forest{for tree [{},c[$abcd$,t][$\bot$,t]]\bb}                     &
  \scriptsize
  \Forest{for tree [{},c[$ab$,t][{},c[$c$,t][$d$,t]]]\bb}             &
  \scriptsize
  \Forest{for tree [{},c[{},c[$a$,t][$b$,t]][{},c[$c$,t][$d$,t]]]\bb} &
  \scriptsize
  \Forest{for tree [{},c [{},c [{},c[a,t][b,t]] [c,t]] [d,t] ] \bb}   &
  \scriptsize
  \Forest{for tree [{},c [{}, c [{}, c
        [a,t][b,t]] [c,t] ] [d,t]] \bb}
\end{tabular}
\end{document}

The output however shows that this does not work

Answer 1

There are two issues.

1. First and foremost, there is a bug in pgf that inserts spurious spaces. A temporary fix of the bug has been provided here. In future versions of pgf, this bug will be fixed.
2. Second, [t] in \begin{tabular}[t]{...} does not top-align the cells in the table. I added a column type T that does that using collcell and adjustbox.

Code:

\documentclass{standalone}
\usepackage{forest}
\usepackage{adjustbox}
\usepackage{collcell}
\makeatletter
% remove the stray space https://tex.stackexchange.com/a/513549
\patchcmd{\pgfutilsolvetwotwoleqfloat}
  { \noexpand\pgfmathfloatdivide@}
  {\noexpand\pgfmathfloatdivide@}
  {}{}
\makeatother
\newcommand{\TopAlign}[1]{\adjustbox{valign=t}{#1}}
\newcolumntype{T}{>{\collectcell{\TopAlign}}c<{\endcollectcell}}

\begin{document}
\forestset{c/.style={circle,draw},t/.style={},}
\def\bb{ \draw[fill=blue,opacity=0.2] (current bounding box.south west) rectangle (current bounding box.north east);}
\def\xx#1#2{
  #1
  \begin{tikzpicture}
    \node [fill=green,opacity=0.2,draw,minimum width=#1cm, minimum height=#2cm] {#1#2};
  \end{tikzpicture}
  #2
}
\begin{tabular}{*{6}{|T}|}
  $(ab.cd)$                                                           &
  $(abcd.\bot)$                                                       &
  $(ab.(c.d))$                                                        &
  $((a.b).(c.d))$                                                     &
  $(a.(b.(c.d)))$                                                     &
  $(((a.b).c).d)$                                                       \\
  \hline
  \xx23                                         &
  \xx34                                                              &
  \xx24                                                             &
  \xx14                                                              &
  \xx41                                                              &
  \xx43                                                                \\
  \hline
  \scriptsize
  \Forest{for tree [{},c[$ab$,t][$cd$,t]]\bb}                         &
  \scriptsize
  \Forest{for tree [{},c[$abcd$,t][$\bot$,t]]\bb}                     &
  \scriptsize
  \Forest{for tree [{},c[$ab$,t][{},c[$c$,t][$d$,t]]]\bb}             &
  \scriptsize
  \Forest{for tree [{},c[{},c[$a$,t][$b$,t]][{},c[$c$,t][$d$,t]]]\bb} &
  \scriptsize
  \Forest{for tree [{},c [{},c [{},c[a,t][b,t]] [c,t]] [d,t] ] \bb}   &
  \scriptsize
  \Forest{for tree [{},c [{}, c [{}, c
        [a,t][b,t]] [c,t] ] [d,t]] \bb}
\end{tabular}
\end{document}