Flutter custom painter: rounded rect and arrow

painter.dart

class _MenuBoxBackground extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final Paint paint = Paint()
      ..color = Colors.white
      ..strokeWidth = 1.W
      ..style = PaintingStyle.fill;
    final double triangleH = 10.H;
    final double triangleW = 25.0.W;
    final double width = size.width;
    final double height = size.height;

    final Path trianglePath = Path()
      ..moveTo(width / 2 - triangleW / 2, height)
      ..lineTo(width / 2, triangleH + height)
      ..lineTo(width / 2 + triangleW / 2, height)
      ..lineTo(width / 2 - triangleW / 2, height);
    canvas.drawPath(trianglePath, paint);
    final BorderRadius borderRadius = BorderRadius.circular(15.W);
    final Rect rect = Rect.fromLTRB(0, 0, width, height);
    final RRect outer = borderRadius.toRRect(rect);
    canvas.drawRRect(outer, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => false;
}

usage.dart

CustomPaint(
  painter: _MenuBoxBackground(),
  child: Container(
    padding: EdgeInsets.only(
        left: 15.W, right: 15.W, bottom: 20.H, top: 20.H),
    child: ...,
  ),
)