user-scroll-device-emulation-frame.html

<!DOCTYPE html>
<html>
  <head>
  </head>
  <body>
    <style>
      body {
        margin: 0px;
      }
      .space {
        height: 1000px;
        width: 1000px;
      }
      .scroller {
        height:500px;
        width: 500px;
        position: relative;
        overflow-y: scroll;
        overflow-x: hidden;
      }
      #topbox {
        border: solid 1px blue;
      }
      #bottombox {
        border:solid 1px black;
        position: absolute;
        top: 500px;
      }
      #overlay {
        pointer-events: none;
        position: absolute;
        height: 1100px;
        width: 500px;
        top: 0px;
        background: rgba(0, 0, 0, 0.5);
      }
    </style>
    <div id="topbox" class="scroller"><div class="space"></div></div>
    <div id="bottombox" class="scroller"><div class="space"></div></div>
    <div id="overlay"></div>
  </body>
</html>

chromium/third_party/blink/web_tests/fast/scrolling/resources/user-scroll-device-emulation-frame.html