Flexible vs Expanded in popup that covers part of the screen in Flutter

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomeWidget(),
    );
  }
}

enum ParentWidgetType {
  expanded,
  flexible,
}

const int itemsCount = 15; // <-- items count in grid

class MyHomeWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: Container(
          color: Colors.grey,
          child: Center(
              child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              RaisedButton(
                onPressed: () => _presentGridPopup(context, ParentWidgetType.expanded),
                child: Text('Expanded'),
              ),
              const SizedBox(height: 5),
              RaisedButton(
                onPressed: () => _presentGridPopup(context, ParentWidgetType.flexible),
                child: Text('Flexible'),
              )
            ],
          )),
        ),
      ),
    );
  }

  void _presentGridPopup(BuildContext context, ParentWidgetType parentWidgetType) {
    Navigator.of(context).push<void>(PageRouteBuilder(
      opaque: false,
      barrierColor: null,
      barrierDismissible: true,
      barrierLabel: 'barrier',
      pageBuilder: (context, animation, secondaryAnimation) => GridPopup(
        parentWidgetType: parentWidgetType,
        itemsCount: itemsCount,
      ),
      transitionDuration: Duration(milliseconds: 300),
      transitionsBuilder: (context, animation, secondaryAnimation, child) => SlideTransition(
        position: Tween<Offset>(begin: Offset(0.0, 1.0), end: Offset.zero).animate(animation),
        child: child,
      ),
    ));
  }
}

class GridPopup extends StatelessWidget {
  final ParentWidgetType parentWidgetType;
  final int itemsCount;

  const GridPopup({
    Key? key,
    required this.parentWidgetType,
    required this.itemsCount,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Align(
        alignment: Alignment.bottomCenter,
        child: Container(
          color: Colors.blue,
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              Padding(
                padding: EdgeInsets.symmetric(vertical: 5),
                child: RaisedButton(
                  child: Text('Close popup'),
                  onPressed: () => Navigator.of(context).pop(),
                ),
              ),
              _buildGrid(),
            ],
          ),
        ),
      ),
    );
  }

  Widget _buildGrid() {
    final _internal = () {
      return GridView.count(
        crossAxisCount: 5,
        crossAxisSpacing: 2.0,
        mainAxisSpacing: 2.0,
        childAspectRatio: 1.0,
        shrinkWrap: true,
        children: List.generate(
          itemsCount,
          (index) => Container(color: Colors.green),
        ),
      );
    };

    switch (parentWidgetType) {
      case ParentWidgetType.expanded:
        return Expanded(child: _internal());
      case ParentWidgetType.flexible:
        return Flexible(child: _internal());
    }
    return Container();
  }
}