placing (on) layers

[Henning Hraban Ramm via ntg-context <ntg-context@ntg.nl>]

Hi again,

I’m always struggling with putting stuff on layers, since these just 
never start where I’d expect them to, and the offsets changed a few 
times within the last years.

Please explain how I should define a layer that covers the whole page, 
since the following MWE is wrong (the layer always starts below the 
first line):


\useMPlibrary[dum]

\setuppapersize[A5][A4]
\setuplayout[
   marking=on,
   location=middle,
]
\definemeasure[Bleed][3mm]
\definemeasure[MaxHeight][{\dimexpr\paperheight + 2\measure{Bleed}\relax}]
\definemeasure[MaxWidth][{\dimexpr\paperwidth + 2\measure{Bleed}\relax}]

\definelayer[papier][
   preset=lefttop,
   %x=0mm,y=0mm,
   width=\paperwidth,
   height=\paperheight,
]
\setupbackgrounds[paper][background=cover,state=start]


\starttext
\showframe
TEST

\setlayerframed[papier][
   preset=lefttop,
   %x=-\measure{Bleed},
   %y=-\measure{Bleed},
   offset=overlay,
   %frame=off,
   width=\measure{MaxWidth},
   height=\measure{MaxHeight},
]{\externalfigure[dummy][
   width=\measure{MaxWidth},height=\measure{MaxHeight}
]}

\flushlayer[papier]

\stoptext
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