shape property
If provided, the shape used for the menu.
If this property is null, then PopupMenuThemeData.shape is used. If PopupMenuThemeData.shape is also null, then the default shape for MaterialType.card is used. This default shape is a rectangle with rounded edges of BorderRadius.circular(2.0).
Example
const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(20))),
Implementation
final ShapeBorder? shape;