<itemvalue="Creates an iOS 14 style selection overlay that highlights the magnified area (or the currently selected item, depending on how you described it elsewhere) of a [CupertinoPicker]. The [background] argument default value is [CupertinoColors.tertiarySystemFill]. It must be non-null. The [capStartEdge] and [capEndEdge] arguments decide whether to add a default margin and use rounded corners on the left and right side of the rectangular overlay. Default to true and must not be null."/>