max-height ignored in Opera?

I have a popup dialog on my site that is position:fixed.

If I set it's height to 80%, then the height of the DIV is 80% the
height of the viewport, which is what I would expect.

If I change this to max-height instead, Firefox limits to 80%, but
Opera seems to totally ignore it and the content overflows off the
bottom of the viewport. Is this a bug in Opera, or am I missing
something?

(This is the latest Opera 9.21 BTW)

Cheers,
Lister