<html>
<head>
  <style>
    #trigger {
      will-change:transform;
    }
    #parent {
      width: 350px;
      height: 450px;
      border: none;
      position: absolute;
      transform: translate(200px, 0px);
    }
  </style>
</head>
<body>
  <div id="trigger"></div>
  <iframe id="parent" src="scrollgrandchild-parent.html"></iframe>
</body>
</html>