浮动DIV

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$(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");   
});