Container is a somewhat "heavy" widget (see #147431), so it'd be great if DecoratedBox could fully decorate a box by itself (as its name would imply).

• PhysicalShape allows setting a clipBehavior; DecoratedBox should as well.
• Decoration objects have intrinsic padding; maybe there should be an option to apply this padding.
• And perhaps the BoxShadow API has some room for improvement, though at this point I'm not sure if there's a clear path forward.