The z-coordinate at which to place the material banner.
This controls the size of the shadow below the material banner.
Defines the banner's Material.elevation.
If this property is null, then MaterialBannerThemeData.elevation of ThemeData.bannerTheme is used, if that is also null, the default value is 0. If the elevation is 0, the Scaffold's body will be pushed down by the MaterialBanner when used with ScaffoldMessenger.
final double? elevation;