Scrollbar thumb is much wider than the scrollbar track in FB4 build i16 when used on mx components. Worked fine in build i15. Causes items in container to shift.

Steps to reproduce:
Create an mx:WindowedApplication, and define a custom Scrollbar skin in the CSS that uses embedded pngs. Create any item that should scroll. You will see that the scrollbar track is the correct size, but the scrollbar thumb is much wider than the track. It should always fit within the track. This causes the edge of the scrolling box to be pushed in, and results in reflow of contained items.

This same scrollbar skin worked fine in FB3, and the identical project in FB3/FB4 build i15 worked fine.

I have enclosed a sample app to show the problem.

Workaround (if any): None.