None uniform borderside container hack example.

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Border Fun',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: const HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    const double width = 220;
    const double height = 150;
    const double borderWidth = 20;
    const double borderRadius = 40;

    return Scaffold(
      appBar: AppBar(title: const Text('Rounded Border Fun')),
      body: const Center(
        child: MikeVersion(
          width: width,
          height: height,
          borderWidth: borderWidth,
          borderRadius: borderRadius,
        ),
      ),
    );
  }
}

// *****************************************************************************

class MikeVersion extends StatelessWidget {
  const MikeVersion({
    Key? key,
    required this.width,
    required this.height,
    required this.borderWidth,
    required this.borderRadius,
  }) : super(key: key);

  final double width;
  final double height;
  final double borderWidth;
  final double borderRadius;

  @override
  Widget build(BuildContext context) {
    return Container(
      width: width,
      height: height,
      decoration: BoxDecoration(
        color: Colors.black,
        borderRadius: BorderRadius.only(
          topRight: Radius.circular(borderRadius),
          bottomRight: Radius.circular(borderRadius),
        ),
      ),
      child: Container(
        margin: EdgeInsets.only(
          top: borderWidth,
          right: borderWidth,
          bottom: borderWidth,
        ),
        decoration: BoxDecoration(
          color: Colors.blue,
          borderRadius: BorderRadius.only(
            topRight: Radius.circular(borderRadius - borderWidth),
            bottomRight: Radius.circular(borderRadius - borderWidth),
          ),
        ),
      ),
    );
  }
}