Sheet interaction isn't notified when another sheet is dismissed using swipe gesture.

[boscojwho]

When two or more modal sheets are presented, and user quickly swipes to close the top sheet, iOS may automatically move the now top-most sheet to a detent it deems appropriate.

When that happens, we need to figure out how to observe that change, and notify SheetInteraction's delegate.

Currently, the delegate isn't notified, and user-interface state may not reflect the actual selected detent.