main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: [
          Positioned.fill(
              child: Center(
            child: Text("Test"),
          )),
          Positioned.fill(
              child: CustomPaint(
            painter: Painter(),
          ))
        ],
      ),
    );
  }
}

class Painter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    Rect r = Rect.fromCenter(center: Offset(100, 100), height: 50, width: 50);

    final leftCorner = Path.combine(
      PathOperation.difference,
      Path()..addRect(r),
      Path()
        ..addRRect(
          RRect.fromRectAndCorners(
            r,
            topRight: Radius.circular(50),
          ),
        ),
    );
    canvas.drawPath(
        leftCorner,
        Paint()
          ..style = PaintingStyle.fill
          ..color = Colors.blue);
  }

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