https://stackoverflow.com/q/55307231/6509751

main.dart

import 'package:flutter_web/material.dart';
import 'package:flutter_web_ui/ui.dart' as ui;

void main() async {
  await ui.webOnlyInitializePlatform();

  double position = 0;

  runApp(
    MaterialApp(
      home: StatefulBuilder(
        builder: (context, setState) => Scaffold(
          body: ScrollDemo(position: (pos) => setState(() => position = pos)),
          floatingActionButton: Padding(
            padding: const EdgeInsets.only(bottom: 48),
            child: FloatingActionButton.extended(
                label: Text('Extent after: $position'), onPressed: () {}),
          ),
        ),
      ),
    ),
  );
}

class ScrollDemo extends StatefulWidget {
  final void Function(double pos) position;

  const ScrollDemo({
    Key key,
    this.position,
  }) : super(key: key);

  @override
  State createState() => _ScrollDemoState();
}

class _ScrollDemoState extends State<ScrollDemo> {
  @override
  void didChangeDependencies() {
    PrimaryScrollController.of(context).removeListener(log);
    PrimaryScrollController.of(context).addListener(log);
    super.didChangeDependencies();
  }

  void log() =>
      widget.position(PrimaryScrollController.of(context).position.extentAfter);

  @override
  Widget build(BuildContext context) => NestedScrollView(
        headerSliverBuilder: (_, __) => <Widget>[
          const SliverAppBar(
            title: Text('Title'),
          )
        ],
        body: RefreshIndicator(
          onRefresh: () => Future.value(null),
          child: CustomScrollView(
            slivers: <Widget>[
              SliverList(
                  delegate: SliverChildListDelegate(<Widget>[
                for (int i = 0; i < 99; i++)
                  ListTile(
                    leading: Text(i.toString()),
                  )
              ])),
            ],
          ),
        ),
      );
}