v0.16.4

Fixed

• False "covered by another widget" warning on a SnackBar action (Android): when several widgets match a
  selector (an exiting SnackBar or route keeps its widgets in the tree briefly) tap now picks the one a pointer can
  actually reach, and a tap only warns after the target has stayed covered for ~0.6 s, so overlays that are still
  sliding in or out are not reported. A really covered target still warns (and costs that extra 0.6 s).