There are two types of elements that (sometimes) prohibit you from positioning a DOM element over them (mostly in Internet Explorer): some form elements and some flash elements. The two options you have are:
to hide these elements when your DOM is going to be over them; this works if you know your DOM element is going to completely obscure that element
element - (mixed, required) A string of the id for an Element or an Element reference that should be shimmed
options - (object, optional) key/value set of options
Options
display - (boolean) display the shim on instantiation; defaults to false
zIndex - (number) the z-index of the shim; optional, default is 1 less than the element
margin - (number) make the iframe smaller than the element to give a buffer (for things like shadows)
offset - (object: {x:#, y:#}) move the iframe up/down, left/right relative to the element
className - (string) className for the shim; defaults to "iframeShim"
browsers - (boolean) allows you to specify the browsers that the iframe should show up for; defaults to ie6 or firefox on a mac (Browser.ie6 || (Browser.firefox && Browser.version < 3 && Browser.Platform.mac)). Example usage: browsers: Browser.ie6 || Browser.opera will show for ie6 and opera
src - (string) this is the source of the Iframe. For the most part, you shouldn't mess with this option. We've tested it across numerous environments (particularly https environments), but if you need to, for some reason, alter it, we've exposed it as an option, as your environment may require you try an alternate string. The default is 'javascript:false;document.write("");'.
Events
onInject - (function) callback executed when the iframe is added to the DOM (which waits until window.onload)
Example
<div id="myFloatingDiv">stuff</div>
var myFloatingDivShim = new IframeShim('myFloatingDiv', {
display: false,
className: 'myFloatingDivShimClass'});
This will hide the iframe shim element. If you don't call this when you hide the element that's over the flash or select list, then that thing will still be obscured.