% Don't break inside a float

% Mock
\let\issue@orig=\cals@issue@row
\renewcommand\cals@issue@row{%
\show\cals@last@context
\issue@orig}

% Start float at the end of page, so that the table breaking code
% think it should split over page.
Start of text\par
\dimen0=\pagegoal\relax
\advance\dimen0 by -5cm\relax
\vskip\dimen0\relax
here we are\par

\newcommand\www{\ht\cals@current@row=2.5cm}

\vbox{%
\begin{calstable}
\colwidths{{0.25in}{1.5in}{1.5in}{1.5in}}
%
\brow \cell{}\cell{C1}\cell{C2}\cell{C3} \www\erow
%
\brow
\cell{R}
\cell{r1c1}
\cell{r1c2}
\cell{r1c3}
\www\erow
%
\brow \nullcell{lrt}
\cell{r2c1}
\cell{r2c2}
\cell{r2c3}
\www\erow
%
\brow
\nullcell{lrb}\spancontent{S}
\cell{r3c1}
\cell{r3c2}
\cell{r3c3}
\www\erow
\end{calstable}
}% vbox
\end{document}
