title: Overflow on div makes it appear above fixed menu
body: <p>I have a fixed menu at the top with a z-index of 999 (to stop anything appearing in front of it in theory). I also have a div that is designed to scroll horizontally. Why are the overflow properties on the scrolling div making it appear on top of the menu bar?</p>
  
  <p>Scrolling div CSS:</p>
  
  <pre><code>.product-viewer {
      width: 85%;
      margin-left: 15%;
      display: inline-block;
      border-top-left-radius: 30px;
      border-top-left-radius: 3vmax;
      border-bottom-left-radius: 30px;
      border-bottom-left-radius: 3vmax;
      height: 300px;
      height: 30vmax;
      background-color: #202020;
      overflow-y: hidden;
      overflow-x: scroll;
      -webkit-overflow-scrolling: touch;
      white-space: nowrap;
      z-index: 0;
  }
  </code></pre>
  
  <p>Menu bar CSS (#menu.fix is when the menu is fixed as it is a "sticky" menu):</p>
  
  <pre><code>#menu {
      width: 100%;
      height: 50px;
      height: 5vmax;
      background-color: rgba(0, 0, 0, 0.8);
      background-blend-mode: hard-light;
      text-align: center;
      overflow: hidden;
      line-height: 50px;
      line-height: 5vmax;
      display: inline-block;
  }
  
  #menu.fix {
      position: fixed;
      left: 0;
      top: 0;
      z-index: 999;
  }
  </code></pre>
  
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