iframe drag behind div

[robert.coup]

Hi All,

I have a page setup like the following:
<div (relative, overflow hidden)>
<div (relative)>
<iframe (absolute)>
</iframe>
</div>
</div>

The idea is that the inner div can be dragged around and the outer div
serves as a window to it. Within the inner div there is content
delivered via iframes. I'm having trouble with the iframes when they
are dragged "offscreen".

In IE6 (surprisingly) it works how I want it to. In Opera8, the iframe
content appears over the top of the body. In Firefox1.5, the same
happens although the outer div border gets drawn all over it (a bit
hard to explain - look for yourself).

My cut-down example is at:
http://www2.onetrackmind.co.nz/iframe_quirk/1.html

I've played with z-indexes on various elements but it doesn't seem to
change anything. I also had a quick hunt through mozilla's bugzilla and
didn't find this issue. Has anyone run into this before or have any
bright ideas?

Thanks in advance,

Rob :)