Steps to Reproduce

1. Execute flutter run on the code sample.
2. Change elevation property of NavigationBar to any value.

Expected results: Different coloring, or shadows.

Actual results: No effect whatsoever.

Reference for @rydmike mentioning his concern on the original PR with this, it might be related to this issue:
#83047 (comment)

Side Note/Suggestion:
I think having no shadow is intentional as per material 3 spec, but the surfaceTint color should get applied to the underlying Material widget based on set elevation, and it should not be fixed to "3", we should have an additional property called surfaceTintColor as in Card widget, and it should be themable too.