<script type="text/javascript">
  $.fn.smartFloat = function () {
    var position = function (element) {
      var top = element.position().top, pos = element.css("position");
      $(window).scroll(function () {
        var scrolls = $(this).scrollTop();
        if (scrolls > top) {
          if (window.XMLHttpRequest) {
            element.css({
              position: "fixed",
              top: 0
            });
          } else {
            element.css({
              top: scrolls
            });
          }
        } else {
          element.css({
            position: pos,
            top: top
          });
        }
      });
    };
    return $(this).each(function () {
      position($(this));
    });
  };
  //绑定
  $("#float").smartFloat();
</script>

<div class="float" id="float" style="border: 1px solid #e0e0e0; padding: 10px;">
  我是置顶时悬浮的区域</div>
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