Strange StartPage in Metapost

[Patrick Gundlach <pg@levana.de>]

Hi out there in the deep Cyberscpace ;-),

--------------------------------myfile.tex----------
\%output=pdf
showframe
\setupcolors[state=start]
\setuppapersize[S6][S6]

\startreusableMPgraphic{DemoFrame}
StartPage ; 
pickup pencircle scaled 2pt ; 
Field[Text][Text] := Field[Text][Text] enlarged .5cm ;
draw (llcorner Field[Text][Text] -- ulcorner Field[Text][Text]);
StopPage ;
\stopreusableMPgraphic

\defineoverlay[DemoFrame]{\reuseMPgraphic{DemoFrame}}
%\setupbackgrounds[page][background=DemoFrame]

\starttext
\dorecurse{10}{\input tufte \par}
\stoptext
------------------------------end----------

when I texexec this, I get strange results.
In my opinion, only the \input tufte should be typeset (and, of course the 
\showframe). But as you might see, the DemoFrame also gets typeset. But I 
did not connect  the overlay to the background. Only when I comment out the 
line with \defineoverlay, the mpgraphic is not drawn. So there are two 
problems here:

1) why is this graphic drawn (even though I have not put it in the 
background)?
2) why is this placed on an incorrect place? (It seems that the page starts 
at the upper left corner of the text field, so the vertical line is shifted 
to the right and down.)

This is pdfeTeX, Version 3.14159-14h-released-20010417-2.1 (Web2C 7.3.3.1)
entering extended mode
(./startpage.tex{/usr/share/texmf/pdftex/config/pdftex.cfg}

ConTeXt  ver: 2001.7.11  fmt: 2001.9.9  int: english  mes: english

Randomizer initialized to 931223677.This is MetaPost, Version 0.641 (Web2C 
7.3.1)
(startpage-mpgraph.mp [1] )

-- 
Viele Grüße, 

    Patrick Gundlach