SliverAppBar extend only on Top (zero offset)

main.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: const Scaffold(
        body: Center(
          child: MyStatefulWidget(),
        ),
      ),
    );
  }
}

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

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}


class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  late ScrollController _scrollController;
  // variable height passed to SliverAppBar expanded height
  late double? _expandedHeight;
  
  // constant more height that is given to the expandedHeight
  // of the SliverAppBar
  final double _moreHeight = 200;
  
  @override
  initState() {
    super.initState();
    // initialize and add scroll listener
    _scrollController = ScrollController();
    _scrollController.addListener(_scrollListen);
    // initially expanded height is full
    _expandedHeight = _moreHeight;
  }
  
  @override
  dispose() {
    // dispose the scroll listener and controller
    _scrollController.removeListener(_scrollListen);
    _scrollController.dispose();
    super.dispose();
  }
  
  _scrollListen() {
    final offset = _scrollController.offset;
    if(offset > _moreHeight) {
      // if offset is more height, disable expanded height
      setState(() {
        _expandedHeight = null;
      });
    } else {
      // if offset is less, keep increasing the height to offset 0
      setState(() {
        _expandedHeight = _moreHeight - offset;
      });
    }
  }
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        controller: _scrollController,
        slivers: <Widget>[
          SliverAppBar(
            pinned: false,
            floating: true,
            expandedHeight: _expandedHeight,
            actions: [
              TextButton(
                onPressed: () {},
                child: const Text('test'),
              ),
            ],
            flexibleSpace: FlexibleSpaceBar(
              // animate the opacity offset when expanded height is changed
              background: AnimatedOpacity(
                opacity: _expandedHeight != null ? _expandedHeight! / _moreHeight : 0,
                duration: const Duration(milliseconds: 300),
                child: const FlutterLogo(),
              ),
            ),
          ),
          SliverToBoxAdapter(
            child: SizedBox(
              height: 2000,
              child: Center(
                child: Container(
                  color: Colors.blue,
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}

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