shows how a border color don't get the opacity

main.dart

import 'package:flutter/material.dart';

void main() {
  final color = Colors.red.withOpacity(0.8);
 
  runApp(
    MaterialApp(
      home: Scaffold(
        body: Center(
          child: Container(
            width: 100,
            height: 100,
            decoration: BoxDecoration(
              color: color,
              borderRadius: BorderRadius.circular(4),
              border: Border.all(
                width: 20,
                color: color,
              ),
            ),
          ),
        ),
      ),
    ),
  );
}