<context source=yes>
\starttext
\blackrule[height=1pt,width=\makeupwidth,]
\startcolumns[rule=on,]
\scale[maxwidth=\textwidth]{
This line avoids the scaled command and will be split using line breaks.
\stopcolumns
\blackrule[height=1pt,width=\makeupwidth,]
\stoptext
</context>