window.onscroll = function(){
    var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
    var waterflow = Node.one(".zebra-yao-waterflow");
    if(scrollTop >= waterflow.offset().top) {
        if(!Node.one(".yao-waterflow-tab").hasClass("position-fixed")) {
            Node.one(".yao-waterflow-tab").addClass("position-fixed");
        }
    } else {
        if(Node.one(".yao-waterflow-tab").hasClass("position-fixed")) {
            Node.one(".yao-waterflow-tab").removeClass("position-fixed");
        }
    }
}

.position-fixed {
  position: fixed;
  top: 0px;
  left: 0px;
}