dynamic-height-1.xhtml

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/"?>
<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
	xmlns:html="http://www.w3.org/1999/xhtml"
	onload="onLoad();">
  <script type="application/javascript">
<![CDATA[
    function onLoad()
    {
      var adjust = document.getElementById("adjust");
      flush();
      adjust.style.height = "5em";
    }

    function flush() { document.documentElement.getBoundingClientRect(); }

]]>
  </script>
  <hbox align="stretch">
    <box id="adjust"/>
    <html:input id="input" value="YO"/>
  </hbox>
</window>