kOpenBorderRadius constant

double const kOpenBorderRadius

Exposes the default border radius for matching iOS 16.0 behavior. This value was eyeballed from the iOS simulator running iOS 16.0.

Below is example code in order to match the default border radius for an iOS 16.0 open preview.
link 
CupertinoContextMenu.builder(
  actions: <Widget>[
    CupertinoContextMenuAction(
      child: const Text('Action one'),
      onPressed: () {},
    ),
  ],
  builder:(BuildContext context, Animation<double> animation) {
    final Animation<BorderRadius?> borderRadiusAnimation = BorderRadiusTween(
      begin: BorderRadius.circular(0.0),
      end: BorderRadius.circular(CupertinoContextMenu.kOpenBorderRadius),
    ).animate(
      CurvedAnimation(
        parent: animation,
        curve: Interval(
          CupertinoContextMenu.animationOpensAt,
          1.0,
        ),
      ),
    );

    final Animation<Decoration> boxDecorationAnimation = DecorationTween(
      begin: const BoxDecoration(
       boxShadow: <BoxShadow>[],
      ),
      end: const BoxDecoration(
       boxShadow: CupertinoContextMenu.kEndBoxShadow,
      ),
     ).animate(
       CurvedAnimation(
        parent: animation,
        curve: Interval(
          0.0,
          CupertinoContextMenu.animationOpensAt,
        ),
      )
    );

    return Container(
      decoration:
        animation.value < CupertinoContextMenu.animationOpensAt ? boxDecorationAnimation.value : null,
      child: FittedBox(
        fit: BoxFit.cover,
        child: ClipRRect(
          borderRadius: borderRadiusAnimation.value ?? BorderRadius.circular(0.0),
          child: SizedBox(
            height: 150,
            width: 150,
            child: Image.network('https://flutter.github.io/assets-for-api-docs/assets/widgets/owl-2.jpg'),
          ),
        ),
      )
    );
  },
)

Implementation

static const double kOpenBorderRadius = _previewBorderRadiusRatio;