How to implement a changing navigation bar?

[Stefan Wachter <Stefan.Wachter@gmx.de>]

Hi all,

I use an overlay to implement a navigation bar in the left margin of a 
document. It seems to me that the processing of the overlay is done only 
once and not for every page again. In addition, the \lastpage is not 
available.

Does anyone know a solution? Maybe headers/footers are a solution. But 
how is absolute positioning accomplished with them?

Thanks for your attention,
--Stefan

PS: Here is my current implementation:

\definelayer[navbar]
\setupbackgrounds[text][leftmargin][background=navbar]
\setlayer[navbar][x=0cm,y=10cm]{\the\pageno}