jsマウススクロールを実現する時、ページの要素が相対的に動かない

        window.onscroll=function(){
            var topScroll = get_scrollTop_of_body();//     ,       
            var bignav = document.getElementById("navbar");//      id
            if(topScroll > 250){ //       250px        
                bignav.style.position = 'fixed';
                bignav.style.top = '0';
                //bignav.style.zIndex = '-1';
            }else{//       250          ，           
                bignav.style.position = 'static';
            }
        }
         /*         */
        function get_scrollTop_of_body(){
            var scrollTop;
            if(typeof window.pageYOffset != 'undefined'){//pageYOffset                
                scrollTop = window.pageYOffset;
            }else if(typeof document.compatMode != 'undefined' && document.compatMode != 'BackCompat')        {
                scrollTop = document.documentElement.scrollTop;
            }else if(typeof document.body != 'undefined'){
                scrollTop = document.body.scrollTop;
            }
            return scrollTop;
        }