t Split</option>
        <option value="topbottom">Top/Bottom Split</option>
      </select></label>
  </div>

  <div id="panes">
    <!-- React test app (shells/dev/app) is injected here -->
    <!-- DevTools backend (shells/dev/src) is injected here -->
    <!-- global "hook" is defined on the iframe's contentWindow -->
    <iframe id="target"></iframe>

    <!-- Draggable divider between panes -->
    <div id="divider"></div>

    <!-- DevTools frontend UI (shells/dev/src) renders here -->
    <div id="d