Fly Down transition broken?

I have a menu that appears from the bottom and disappears there too.

When appearing, there is a scroll bar that appears on the browser while it slides (not too bad)

When disappearing, the image disappears outside the area and the browser is now eternally stuck with the scoll bar and the extra zone at the bottom. Whenever the object is called again, it doesn't animate properly and appears under the presentation.

Anyone know of a way to render this correctly?