[Deff]

<script src="http://code.jquery.com/jquery-latest.js"></script>

<input id="toggle" type="button" value="toggle">
<div style="position:relative;">
<iframe name="ff2" src="" height=100%  width=100% style="z-ndex:-100;position:absolute"  border="0" scrolling="no"/></iframe>
<div id="my" style="background-color:blue; min-height:34px;min-width:34px;"></div>
</div>




<script type="text/javascript">
$('#toggle').click(function() {
 $('#my').toggle()
});
function tstDiv() {
   alert($('#my').is(':visible')?"Показан":"Cкрыт");
}
</script>

<script type="text/javascript">
// Тестируем смену размера iframe
var timeoutId;
$(window).load(function() {
 ff2.onresize = function() {
   clearTimeout(timeoutId)
   timerId = setTimeout(tstDiv,12);
 }
});
</script>