title: Fixed header disappear when scrolling down in webview in iOS 11
body: <p>I have a native iOS app with a webview to display web content.
  I have a fixed header in my app with the following properties:</p>
  
  <pre><code>#header {
      height: 60px;
      background-color: @mainColor;
      color: #ffffff;
      padding: 10px;
      text-align: center;
      position: fixed;
      width: 100%;
      z-index: 1;
  }
  </code></pre>
  
  <p>Before I upgraded to iOS 11 everything worked fine. Now when I scroll down/up the header disappears during the scroll, and when the scroll is done, the header appears again.</p>
  
  <p>This can also be reproduced in Xcode 8.</p>