Demo app for GlobalKey research

main.dart

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  var _colored = false;

  @override
  Widget build(BuildContext context) {
    const counterWidget = _StatefulCounter();
    final child = _colored
        ? ColoredBox(color: Colors.blue[50]!, child: counterWidget)
        : counterWidget;
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('GlobalKey practice')),
        body: SizedBox.expand(child: child),
        floatingActionButton: FloatingActionButton(
          onPressed: () => setState(() => _colored = !_colored),
          child: const Icon(Icons.change_circle_outlined),
        ),
      ),
    );
  }
}

class _StatefulCounter extends StatefulWidget {
  const _StatefulCounter();

  @override
  State<_StatefulCounter> createState() => _StatefulCounterState();
}

class _StatefulCounterState extends State<_StatefulCounter> {
  var _count = 0;

  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        Text('$_count', style: const TextStyle(fontSize: 24)),
        const SizedBox(width: 32),
        IconButton(
          onPressed: () => setState(() => _count++),
          icon: const Icon(Icons.add),
        ),
      ],
    );
  }
}