Flutter: how to animate transition between expanded and normal

Question

I need to animate the size of a widget when the widget changes from an expanded widget to a normal widget inside a Flex. I also want the other children of the flex to animate to their new positions.

The current code doesn't work because it doesn't animate a jumps between the sizes:

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

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool expand = false;

  void _expandToggle() {
    setState(() => expand = !expand);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'Whether the child below is expanded:',
            ),
            Expanded(
              flex: expand ? 1 : 0,
              child: AnimatedSize(
                duration: const Duration(seconds: 3),
                child: ColoredBox(
                  color: Colors.yellow,
                  child: Text(
                    '$expand',
                    style: Theme.of(context).textTheme.headlineMedium,
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _expandToggle,
        child: const Icon(Icons.add),
      ),
    );
  }
}

I think the reason why it doesn't animate is because ideally the Expanded should be wrapped in the AnimatedSize. But of course Flutter wouldn't allow that.