An example of a site using it (resize your window while viewing it to appreciate the behaviour).
Upon upgrading my Opera-browser from version 9.63 to 10.11, I noticed that these suckerfish-based menus misbehave under Opera-10.11 -- the dropdowns fail to go away when they should, although they do on a Reload. I'm not aware of any other browser having that problem, but will attempt to solve it for Opera-10. Googling for a solution wasn't fruitful; looks as though other people's suckerfish-style menus are working in Opera-10, in fact one person states that Opera-10 is the first version of Opera for which his work correctly.
These menus still work as desired in Firefox-1, Firefox-2, Firefox-3 (after "KLUGE-3.6"), Opera-9, ie6 (under wine), ie7 (under wine). They misbehave in Konquerer in what looks to be a worse case of the Firefox-3 problem, not the Opera-10 problem.
Shortly after I'd gone to Opera-10.11 another new version, 10.60, became available, and I promptly upgraded to it. However it was such a disaster that I felt forced to go back to an earlier version, and going back to 9.63 was just as easy as going to 10.11 so that's what I did, and thus getting these "suckerfish" menus working in Opera-10 lost its urgency.