https://stackoverflow.com/questions/75364430/how-can-i-smoothly-scroll-between-nestedscrollview-and-listview-builder?noredirect=1#comment133004515_75364430

main.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({
    Key? key,
  }) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  late ScrollController _scrollController;
  
  @override
  void initState() {
    super.initState();
    
    _scrollController = ScrollController();
  }
  
  @override
  void dispose() {
    _scrollController.dispose();

    super.dispose();
  }
  
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        child: const Text('Go to top'),
        onPressed: () => _scrollController.animateTo(
          0, 
          duration: const Duration(seconds: 2), 
          curve: Curves.easeInOut,
        ),
      ),
      body: CustomScrollView(
        // Controller: attached to both: 
        // - SliverAppBar and SliverList.
        // + any other sliver you provide in the [slivers] param.
        controller: _scrollController, 
        slivers: [
          const SliverAppBar(expandedHeight: 400),
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (context, index) {
                return Center(child: Text('item $index.'));
              },
            ),
          ),
        ],
      ),
    );
  }
}