min_width_row.dart

import 'dart:math';

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

const redMaxWidth = 600.0;
const redMinWidth = 300.0;
const blueMaxWidth = 200.0;

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: LayoutBuilder(
          builder: (context, constraints) {
            final rowWidth = constraints.maxWidth;
            final blueWidth = max(0.0, min(blueMaxWidth, rowWidth - redMinWidth));
            return Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: [
                const Flexible(
                  child: MyWidget(color: Colors.red, width: redMaxWidth),
                ),
                MyWidget(color: Colors.blue, width: blueWidth),
              ],
            );
          } 
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  final Color color;
  final double width;

  const MyWidget({Key? key, required this.color, required this.width}) : super(key: key);
  
  @override
  Widget build(BuildContext context) {
    return Container(
      decoration: BoxDecoration(
        color: color,
        borderRadius: BorderRadius.circular(8.0),
      ),
      height: 100.0,
      width: width,
      child: SizeOverlay()
    );
  }
}

class SizeOverlay extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(builder: (context, constraints) {
      return SizedBox.expand(
        child: Align(
          child: Text(
            '${constraints.maxWidth}',
            style: const TextStyle(fontSize: 24.0),
          ),
        ),
      );
    });
  }
}