This defines the behavior and location of the snack bar.
Defines where a SnackBar should appear within a Scaffold and how its
location should be adjusted when the scaffold also includes a
FloatingActionButton or a BottomNavigationBar
If this property is null, then SnackBarThemeData.behavior of
ThemeData.snackBarTheme is used. If that is null, then the default is
If this value is SnackBarBehavior.floating, the length of the bar
is defined by either width or margin.
final SnackBarBehavior? behavior;