On 6/1/2014 5:40 PM, Rik wrote:

Is this also the cause of problems I noted with heading placed into the
margin, as in the example below?

no, unrelated, try this:

\setuphead
  [section]
  [after={\strut\blank[-2*line,samepage,disable]},
   command=\SectionCommand,
   distance=0pt]

\define[2]\SectionCommand{\inleft{XXXXXX}}

\define[2]\MarginHead{\inmargin{#1\\#2}}
\setuphead    [section]
              [command=\MarginHead,
               distance=0pt,
               after={\strut\blank[-2*line,samepage,disable]}]
%% With no \setupwhitespace, or any \setupwhitespace setting that
%%   results in zero interparagraph whitespace, all is well.
%setupwhitespace      [none]%    okay
%setupwhitespace      [fix]%     okay
%% With non-zero interparagraph whitespace, the heading separates
%%   from the text
\setupwhitespace      [line]%    bad
\starttext
\startchapter[title={Weisman}]
 \input weisman\par
 \input weisman
\startsection[title={Thuan}]
 \input thuan\par
 \input thuan
\stopsection
\stopchapter
\stoptext