Keeping state when widget is repositioned

main.dart

import 'package:flutter/material.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool _isBaz = true;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Foo(child: Counter(), isBaz: _isBaz),
      floatingActionButton: FloatingActionButton(
        onPressed: () => setState(() => _isBaz = !_isBaz),
        tooltip: 'Toggle isBaz',
        child: const Icon(Icons.ac_unit),
      ),
    );
  }
}

class Counter extends StatefulWidget {
  @override
  State<Counter> createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
      floatingActionButton: FloatingActionButton(
        onPressed: () => setState(() => _counter += 1),
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

class Foo extends StatefulWidget {
  final Widget child;
  final bool isBaz;

  const Foo({required this.child, required this.isBaz});

  @override
  State<Foo> createState() => FooState();
}

class FooState extends State<Foo> {
  final _key = GlobalKey();

  @override
  Widget build(BuildContext context) {
    // Removing the `_key` or the `KeyedSubtree` causes `child` to be 
    // disposed and recreated when `isBaz` changes.
    if (widget.isBaz) {
      return KeyedSubtree(key: _key, child: widget.child);
    }
    return Container(child: KeyedSubtree(key: _key, child: widget.child));
  }
}