On 2013–09–17 Sanjoy Mahajan wrote:

> Is this a bug, or just a change in setuphead that I didn't take
> account of?

It is a bug. ConTeXt sometimes uses the rightmargindistance for the
left margin, which would be correct for a double sided layout, but
is definitely wrong for a single sided document. However, even in a
double sided layout the calculations are not correct. Here a smaller
example. Note that the output is wrong either way, with
\setuppagenumbering commented out or not.

%% \setuppagenumbering[alternative=doublesided]
\setuplayout[rightmargindistance=1cm]
\setuphead[section][alternative=inmargin]
\showframe

\starttext
  \dorecurse{16}{\section{Blah}
    \input tufte}
\stoptext

Marco