当我在某个点后滚动时,如何使div滚动?

[ Working demo ]

var el  = $("#sticky");
var win = $(window);
var width  = el.width();
var height = el.height();
var win_height = $(window).height();

window.onscroll = function() {
  var offset = el.offset().top + height - win_height;
  if ( win.scrollTop() > offset ) {
    window.onscroll = function() { 
      el.css({
        width: width,
        position: "absolute",
        top: win.scrollTop() + win_height - height
      });
    };  
  }
};

position: "fixed"
bottom: 0