Use case

Currently, the BottomNavigationBar documentation supplies sample code for a simple example involving updating the displayed Scaffold.body widget. However, there can also be more unique interaction patterns, such as:

1. tapping an already-selected index for additional behavior (ie. to scroll to top of displayed page)
2. having a unique item that doesn't update the selectedIndex (ie. a button that opens a dropdown or modal instead of updating the displayed Scaffold.body)

While these are less common, there is some desire for instruction on getting these behaviors implemented.

Proposal

Create sample docs for one or both cases above to show that BottomNavigationBar can be more flexible than expected.