Hi Hans,
In the document Metafun, you use commands that I like, and I try to understand the definition and operation, which is not always easy with TeX commands.
Also, in the following code, this does not give the expected result.
Thank you.

Fabrice

\showframe

\setupcolors
  [state=start]

\setuppagenumbering[location=,alternative]

\startuniqueMPgraphic{titled}
path p ;
p := unitsquare xscaled OverlayWidth yscaled OverlayHeight ;
pickup pencircle scaled 1mm ;
drawoptions(withcolor .625yellow) ;
draw llcorner p--lrcorner p ;
setbounds currentpicture to p ;
\stopuniqueMPgraphic

\defineoverlay[titled][\uniqueMPgraphic{titled}]

\def\ChapterCommand#1#2%
  {\setbox0=\hbox to \leftmarginwidth{#1\hss}%
   \hskip-\leftmarginwidth
   \hskip-\leftmargindistance
   \framed
     [background=titled,frame=off,offset=0pt]
     {\box0
      \hskip\leftmargindistance
      #2}}

\setuphead
  [chapter,section,subsection]
  [command=\ChapterCommand,
   color=darkred]

\starttext
\chapter{First}
\section{First}
\subsection{First}
\stoptext