If non-null, the corners of this box are rounded by this BorderRadius.
Applies only to boxes with rectangular shapes; ignored if shape is not BoxShape.rectangle.
The shape or the borderRadius won't clip the children of the decorated Container. If the clip is required, insert a clip widget (e.g., ClipRect, ClipRRect, ClipPath) as the child of the Container. Be aware that clipping may be costly in terms of performance.
final BorderRadiusGeometry? borderRadius;