It's been very useful to have a version of mx:ViewStack for Spark that has a dataProvider. It's just a SkinnableDataContainer (or such) + StackLayout.
Would Royale want a Spark StackLayout? Apache Flex has an experimental StackLayout, but it's unbelievably complicated with lots of flash imports. A straightforward (and short) implementation of StackLayout was fairly easy for Flex, and I guess would be for Royale, too.
