Fabrice Couvreur schrieb am 27.08.2021 um 10:04:
> Hi Wolfgang,
> This may not be nicest but seems to work
> Fabrice
>
> %%%%%%%%%%%%%%%%%%%%%%
>
> \definelayout
> [1]
>   [header=10mm]
>
> \setuplayout
>   [header=0pt,
>  height=middle]
>
> \setuppagenumbering
> [alternative=doublesided,location=]
>
> \setupbackgrounds
> [header]
>   [text]
>   [frame=off,bottomframe=on,
>  framecolor=lightgray,rulethickness=1pt]
>
> \startsetups[header]
> \doifelse{\totalnumberofpages}{1}
>     {\setuplayout[1]}
> {\setuplayout[reset]}
> \stopsetups
>
>
> \starttext
> \input knuth
> %\dorecurse{10}{\input knuth\par}
> \stoptext

I have no idea what you try to achieve but the check for the number of 
pages in your document is never used. The layout on the first page is 
different from the following pages but this is just how \definelayout 
work when you use a number as first argument.


What you have to understand is that a setups-environment is just a macro 
definition where you provide the names as argument to the start command. 
This means

     \startsetups [header]
     ...
     \stopsetups

works the same way as

     \def\internalname_header
       {...}

but you have to use

     \setups [header]

rather than

     \internalname_header

to get the content of the environment.


Wolfgang