No scroll on add items on top of SliverList

main.dart

import 'package:flutter/material.dart';

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

/// This Widget is the main application widget.
class MyApp extends StatelessWidget {
  static const String _title = 'No scroll on add items on top of SliverList';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: MainWidget(),
    );
  }
}

class MainWidget extends StatefulWidget {
  MainWidget({Key key}) : super(key: key);

  @override
  _MainWidgetState createState() => _MainWidgetState();
}

class _MainWidgetState extends State<MainWidget> {
  List<int> top = [];
  
  @override
  Widget build(BuildContext context) {
    const Key centerKey = ValueKey('anchor');
    return Scaffold(
      appBar: AppBar(
        title: const Text('Press on the plus to add items above'),
        leading: IconButton(
          icon: const Icon(Icons.add),
          onPressed: () {
            setState(() {
              top.add(top.length);
            });
          },
        ),
      ),
      body: CustomScrollView(
        center: centerKey,
        slivers: <Widget>[
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (BuildContext context, int index) {
                return Container(
                  alignment: Alignment.center,
                  color: Colors.blue[200 + top[index] % 4 * 100],
                  height: 100 + top[index] % 4 * 20.0,
                  child: Text('Item: -${index + 1}'),
                );
              },
              childCount: top.length,
            ),
          ),
          SliverToBoxAdapter(
            key: centerKey,
            child: Text("New items above"),
          ),
        ],
      ),
    );
  }
}