%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-3-29 on page I-355 in "Actions based on the current column".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{243.0pt}

  \setlength\textheight{12\baselineskip}

  % small headings for the example
  \makeatletter
  \renewcommand\section{\@startsection{section}{1}{\z@}%
    {-1.25ex \@plus-1ex \@minus-.2ex}%
    {.5ex}%
    {\normalfont\large\bfseries}}
  \makeatother

%StartShownPreambleCommands
\usepackage{lipsum,ragged2e}
\usepackage[colaction]{multicol}
\newcommand\lcolsection{\par
  \docolaction*{}{}{\columnbreak}%
  \section}
%StopShownPreambleCommands

\begin{document}
\begin{multicols}{2} \RaggedRight
  \lcolsection{A} \lipsum[1][1-2]
  \lcolsection{B} \lipsum[2][1-4]
  \lcolsection{C} \lipsum[3]
\end{multicols}
\end{document}