<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
<window id="yourwindow"
        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        xmlns:html="http://www.w3.org/1999/xhtml">
<vbox align="left">
<iframe style="border: 1px solid black; width: 200px; height: 200px; visibility:hidden"/>
<hbox style="width:200px; height:100px; background:white;">
  <hbox style="width:100px; height:100px; background:cyan;" flex="0"></hbox>
</hbox>
<hbox style="width:200px; height:100px; background:white;">
</hbox>
</vbox>
</window>
