% Wrong behaviour:
% - page 1: "a"
% - page 2: thead, tbody1
% - page 3: thead, tbody2, tbody3
% Right behaviour:
% - page 1: "a"
% - page 2: thead, tbody1, tbody2, tbody3
%
%
\edef\oldoutput{\the\output}
\output={\immediate\show O%
\oldoutput}
\let\oldifbreak=\cals@ifbreak
\def\cals@ifbreak{%
\oldifbreak\iftrue \immediate\show T\else \immediate\show F\fi
\cals@tmp}
%\tracingoutput=1
%\tracingpages=1
%
%
%
a\par
\dimen0=\pagegoal\relax
\advance\dimen0 by -\pagetotal\relax
\advance\dimen0 by -150pt
\vskip\dimen0\relax
\begin{calstable}
\colwidths{{100pt}{100pt}}
\thead{\brow\cell{thead}\cell{\vrule width10pt height100pt depth0pt}\erow}
\brow\cell{tbody1}\cell{\vrule width10pt height100pt depth0pt}\erow
\brow\cell{tbody2}\cell{\vrule width10pt height100pt depth0pt}\erow
\brow\cell{tbody3}\cell{\vrule width10pt height100pt depth0pt}\erow
\end{calstable}