It turns out that the extra whitespace before and after the section headers
in the body appears when I put \startcolumns[n=2] before the body text and
\stopcolumns after it.  Here's the MWE, just slightly modified from your
example:

%%%% begin example
\setuphead
   [chapter,section,subsection,subsubsection]
   [before={\blank[nowhite]},
    after={\blank[nowhite]}]

\setupwhitespace
   [medium]

%\showgrid

\starttext

\startcolumns[n=2]

\completecontent

\chapter{Chapter}

\samplefile{knuth}

\section{Section}

\samplefile{weisman}

\subsection{Subsection}

\samplefile{weisman}

\subsubsection{Subsubsection}

\samplefile{weisman}

\stopcolumns

\stoptext
%%%% end example

On Sun, Mar 22, 2020 at 10:07 PM Wolfgang Schuster <
wolfgang.schuster.lists@gmail.com> wrote:

> T. Kurt Bond schrieb am 22.03.2020 um 23:37:
> > I got LMTX running on macOS Catalina.  I followed the directions here
> > <https://wiki.contextgarden.net/Installing_ConTeXt_LMTX_on_MacOS> for
> > versions prior to Catalina, including authorizing the mtxrun script
> > through Security & Privacy General tab.  The first run of install.sh
> > stopped with the errors:
> >
> >     cp: cannot stat
> >
>  '/Users/tkb/context-osx-64/tex/texmf-context/scripts/context/lua/mtxrun.lua':
> >     No such file or directory
> >     cp: cannot stat
> >
>  '/Users/tkb/context-osx-64/tex/texmf-context/scripts/context/lua/mtx-install.lua':
> >     No such file or directory
> >
> >
> > So I ran install.sh again.  This time it said a bunch of things were
> > missing and downloaded them.  It seemed to finish ok, so I ran context
> > on my document and it produced a PDF!  (The ConTexT with TeXLive 2018
> > and TeXLive 2019 both died on this document with different unhelpful
> > errors, by the way.)
> >
> > Both LMTX and TeXLive 2019 ran 5 to 10 times slower than TeXLive 2018,
> > and both complain about "fuzzy paragraphs" with a list of U+XXXX
> > codepoints.  What's that mean?
> >
> > However, the PDF has unwanted whitespace in many areas.
> >
> >   * It has unwanted whitespace between the entries in the Table of
> >     Contents.
> >   * It has unwanted whitespace before and after section headings,
> >     despite me having "before={},after={}" in the \setuphead commands
> >     for all the levels of sections.
> >   * And it had a large unwanted white space between paragraphs until I
> >     \setupwhitespace[none]. I'd prefer a little bit of white space
> >     between paragraphs, but I tried \setupwhitespace[0.5ex] and
> >     \definemeasure[between][0.25ex] with
> >     \setupwhitespace[\measure{between}], and \setupwhitespace[5pt] but
> >     all those went back to way too much white space between paragraphs.
> >
> > Any ideas?
>
> Do you have a working minimal example for the problem because when I run
> the following example on my machine there is no extra whitespace.
>
> %%%% begin example
> \setuphead
>    [chapter,section,subsection,subsubsection]
>    [before={\blank[nowhite]},
>     after={\blank[nowhite]}]
>
> \setupwhitespace
>    [medium]
>
> %\showgrid
>
> \starttext
>
> \completecontent
>
> \chapter{Chapter}
>
> \samplefile{knuth}
>
> \section{Section}
>
> \samplefile{weisman}
>
> \subsection{Subsection}
>
> \samplefile{weisman}
>
> \subsubsection{Subsubsection}
>
> \samplefile{weisman}
>
> \stoptext
> %%%% end example
>
> Wolfgang
>
>

-- 
T. Kurt Bond, tkurtbond@gmail.com