OK... I couldn't let Martin top me here :)
The sample he gives works well for most cases, but in Firefox the "empty" parts of the dynamic panel will prevent the events from firing on widgets underneath it. For example in the collapsed state on the dynamic panel in the above sample, if there is a button under the dynamic panel, clicking it will not work even though it appears as if there is nothing above it.
The way to resolve this is the use the show and hide actions for the dynamic panel like in the attached sample. Mouse enter on a menu button shows the hidden dynamic panel containing the menu and mouse enter on the image map region surrounding the menu in the dynamic panel hides it.
In this sample, a neighboring menu item was added and you can see that they will both work in the same way without any additional effort except to make sure the dynamic panels with the menus are on top of the menu items.
This is going to need a walkthrough in a blog post...