I want to set the background of the entire header (from left edge of
the paper to the right edge). As far as I could tell, there is no way
to coax setupbackgrounds to do this. I can do this using layers or
abuse the margin and offset keys of framed to make it cover the
remaining area, but is there really no direct way to do this? Here is
my attempt so far.
\setuppapersize [S6] [S6]
%%%% What should go here???
\starttext test \stoptext