<?xml version="1.0"?>
<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        class="reftest-wait"
        onload="tweak()">
  <style xmlns="http://www.w3.org/1999/xhtml">
    <![CDATA[
    panelview {
      border: 1px solid black;
      background: green;
      display: flex;
      height: 50px;
    }
    ]]>
  </style>
  <script>
    <![CDATA[
    function tweak() {
      var tweakMe = document.getElementById("tweakMe");
      tweakMe.style.width = "100px";
      document.documentElement.className = "";
    }
    ]]>
  </script>
  <hbox>
    <panelview id="tweakMe"></panelview>
  </hbox>
</window>