HTML5视频标签在不使用poster的情况下无法在移动Safari上显示缩略图

<video controls poster="thumbnail.jpg">
  <source src="video.mp4" type="video/mp4">
</video>

<video id="myVideo" controls>
  <source src="video.mp4" type="video/mp4">
</video>

<script>
  var video = document.getElementById('myVideo');
  video.addEventListener('loadeddata', function() {
    var canvas = document.createElement('canvas');
    canvas.width = video.videoWidth;
    canvas.height = video.videoHeight;
    canvas.getContext('2d').drawImage(video, 0, 0, canvas.width, canvas.height);
    var thumbnailURL = canvas.toDataURL();
    video.setAttribute('poster', thumbnailURL);
  });
</script>