CustomScrollView with multiple silvers, expand first Widget to bottom with key set

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    home: TestPage(),
  ));
}

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

  @override
  State<TestPage> createState() => _TestPageState();
}

class _TestPageState extends State<TestPage> {
  List<Widget> newList = List.generate(
    20,
    (index) => Text('Upper ${index.toString()}'),
  );
  List<Widget> myList = List.generate(
    20,
    (index) => Text('Lower ${index.toString()}'),
  );

  final Key centerKey = const ValueKey('second-sliver-list');
  final scrollController = ScrollController();
  final keyTop = GlobalKey();

  @override
  void dispose() {
    scrollController.dispose();
    super.dispose();
  }

  @override
  void initState() {
    super.initState();
  }

  bool _isExpanded = false;

  @override
  Widget build(BuildContext context) {
    String productDescription =
        "Peanut butter, a creamy concoction crafted from roasted peanuts, is a timeless culinary delight cherished by millions worldwide. Its rich, nutty flavor and smooth texture make it a versatile ingredient that transcends traditional boundaries, finding its way into sandwiches, desserts, sauces, and even savory dishes. This delectable spread has captured the hearts and palates of food enthusiasts for generations, evolving from a humble pantry staple to a beloved icon of gastronomy. In this exploration, we delve deep into the captivating world of peanut butter, uncovering its history, culinary uses, nutritional benefits, and enduring appeal. Peanut butter, a creamy concoction crafted from roasted peanuts, is a timeless culinary delight cherished by millions worldwide. Its rich, nutty flavor and smooth texture make it a versatile ingredient that transcends traditional boundaries, finding its way into sandwiches, desserts, sauces, and even savory dishes. This delectable spread has captured the hearts and palates of food enthusiasts for generations, evolving from a humble pantry staple to a beloved icon of gastronomy. In this exploration, we delve deep into the captivating world of peanut butter, uncovering its history, culinary uses, nutritional benefits, and enduring appeal.";

    return Scaffold(
      floatingActionButton: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        children: [
          FloatingActionButton.extended(
            onPressed: () {
              setState(() {
                newList.add(Text('Upper ${newList.length}'));
              });
            },
            label: const Text('Add to Upper'),
          ),
          const SizedBox(height: 10),
          FloatingActionButton.extended(
            onPressed: () {
              setState(() {
                myList.add(Text('Lower ${newList.length}'));
              });
            },
            label: const Text('Add to Lower'),
          ),
        ],
      ),
      appBar: AppBar(),
      body: CustomScrollView(
        center: centerKey,
        controller: scrollController,
        slivers: [
          SliverToBoxAdapter(
            key: keyTop,
            child: Column(
              children: [
                Image.network(
                    "https://m.media-amazon.com/images/I/719NQBiqh9L.jpg"),
                SizedBox(height: 16.0),
                Text(
                  'Product Name',
                  style: TextStyle(fontSize: 20, fontWeight: FontWeight.bold),
                ),
                SizedBox(height: 8.0),
                Text(
                  '\$100', // Replace with actual price
                  style: TextStyle(fontSize: 16),
                ),
                SizedBox(height: 16.0),
                AnimatedContainer(
                  duration: Duration(milliseconds: 500),
                  curve: Curves.easeInOut,
                  height: _isExpanded
                      ? null
                      : 85.0, // Adjust height to show around 3-4 lines
                  child: Text(
                    productDescription,
                    style: TextStyle(fontSize: 16),
                    overflow: TextOverflow.fade,
                  ),
                ),
                SizedBox(height: 16.0),
                GestureDetector(
                  onTap: () {
                    setState(() {
                      _isExpanded = !_isExpanded;
                    });
                  },
                  child: Text(
                    _isExpanded ? 'Read less' : 'Read more',
                    style: TextStyle(color: Colors.blue),
                  ),
                ),
              ],
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (context, index) {
                return ListTile(
                  title: newList[index],
                );
              },
              childCount: newList.length,
            ),
          ),
          SliverList(
            key: centerKey,
            delegate: SliverChildBuilderDelegate(
              (context, index) {
                return ListTile(title: myList[index]);
              },
              childCount: myList.length,
            ),
          ),
        ],
      ),
    );
  }
}