在iframe中滚动时，修复屏幕顶部的div

#topDiv {
  position: fixed;
  top: 0;
  width: 100%;
  background-color: #fff;
  z-index: 9999;
}

var iframe = document.getElementById('myIframe');
iframe.contentWindow.onscroll = function() {
  var scrollTop = iframe.contentWindow.pageYOffset || iframe.contentDocument.documentElement.scrollTop;
  var topDiv = document.getElementById('topDiv');
  if (scrollTop > 0) {
    topDiv.style.top = scrollTop + 'px';
  } else {
    topDiv.style.top = '0';
  }
};