Stop scrolling one mc

I have two mc s. One is small and another one is long long. At first only one mc is in stage. This one is menu. I want to make thisway that when user scrolls then this menu scrolls up, but it should stop when hal of it is hidden to url bar /(out from stage) This long mc should be scrollable all the time. And when i scroll back i want that this first mc will scroll also back to center.
Thanks.