buildWithChild method
A build method that receives an extra child parameter.
This method may be called with a child different from the parameter
passed to the constructor of SingleChildStatelessModifier.
It may also be called again with a different child, without this widget
being recreated.
Implementation
@override
Widget buildWithChild(BuildContext context, Widget? child) {
  return CupertinoScrollbar(
    key: modifierKey,
    controller: controller,
    isAlwaysShown: isAlwaysShown,
    notificationPredicate: notificationPredicate,
    radius: radius ?? CupertinoScrollbar.defaultRadius,
    radiusWhileDragging: radiusWhileDragging,
    scrollbarOrientation: scrollbarOrientation,
    thickness: thickness ?? CupertinoScrollbar.defaultThickness,
    thicknessWhileDragging: thicknessWhileDragging,
    thumbVisibility: thumbVisibility,
    child: child!,
  );
}