function adjustHeader()
{
	// exchange body top margin by equally-sized header-padding element
	var padding = document.getElementById( "padding" );
	if( padding.style.height == "" )
	{
		var bodyStyles = ( window.getComputedStyle )? getComputedStyle( document.body, null ) : document.body.currentStyle;
		padding.style.height = bodyStyles.marginTop;
		padding.style.backgroundColor = ( window.bgColor )? window.bgColor : "white";
		padding.style.display = "";
		document.body.style.marginTop = "0px";
	}
	// expand underlay to header's height
	var header = document.getElementById( "header" );
	var underlay = document.getElementById( "underlay" );
	underlay.style.height = header.offsetHeight+ "px";
	underlay.style.display = "";
	header.style.position = "fixed";
	return;
}

