浮动DIV

$(window).bind("scroll", function test() {  
var bodyTop = 0;  
    if (typeof window.pageYOffset != 'undefined') {  
    bodyTop = window.pageYOffset;  
}  
else if (typeof document.compatMode != 'undefined' && document.compatMode != 'BackCompat')  
{  
    bodyTop = document.documentElement.scrollTop;  
}  
else if (typeof document.body != 'undefined') {  
    bodyTop = document.body.scrollTop;  
}  
var num = 0 + bodyTop;  
$("#submenu").css("top",  num + "px");  
});