Are you sure your viewport has a scroll event?
Do you have a simple test case of this not working? You may need to listen for a change event on the VScrollBar and/or HScrollBar.
When I add the event listner (see above) shouldn't that event be getting dispatched when i move the vertical scroll bar?
My list definately has more than enough elements and does have a scroll bar vertically.
Well, technically you can add an event listener for any old event (whether it exists or not) and I dont believe that Flex will complain. But unless something actually dispatches that event the event handler will never get called. I mean, you could change the "scroll" event to say "peanuts" and I'm sure it'd do the same thing (ie: nothing).
For a probably semi-obsolete example of how I did this in an earlier Flex 4.0 build, check out http://blog.flexexamples.com/2009/05/31/detecting-when-the-vertical-scroll-bar-is-scrolled -on-a-spark-list-control-in-flex-4/.
PS: One day I'll update some/all of these old Flex 4.0 examples, I promise*.
* - maybe.
Also, I believe this is essentially the same issue, http://bugs.adobe.com/jira/browse/SDK-21357; "Spark Scroller should dispatch a change/scroll event once the scroll position of one of the scrollbars have changed".
Feel free to add your comments or vote on the issue if you agree. Or if you disagree, feel free to add that to the bug comments as well.
ah ok so this is the key here:
And yes I would say my vote is to include a scroll dispatched event property on the List control.