flutter_scrollbar_performance_spike - Demonstrates performance issues detailed here: https://github.com/flutter/flutter/issues/52207

gistfile1.txt

import 'package:flutter/material.dart';

class ScrollbarSpike extends StatefulWidget {
  @override
  _ScrollbarSpikeState createState() => _ScrollbarSpikeState();
}

class _ScrollbarSpikeState extends State<ScrollbarSpike> {
  static List<String> items = List.generate(10000, (index) => "List Item: $index");

  ScrollController scrollController;

  @override
  void initState() {
    scrollController = ScrollController();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    double scrollerWidth = 20;
    return Stack(
      children: <Widget>[
        Padding(
            padding: EdgeInsets.only(right: scrollerWidth),
            child: ListView.builder(
                physics: AlwaysScrollableScrollPhysics(),
                itemCount: items.length,
                controller: scrollController,
                itemBuilder: (_, index) => Text(items[index]))),
        Scrollbar(width: scrollerWidth, controller: scrollController)
      ],
    );
  }
}

class Scrollbar extends StatefulWidget {
  final double width;
  final ScrollController controller;

  const Scrollbar({Key key, this.width, this.controller}) : super(key: key);

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

class _ScrollbarState extends State<Scrollbar> {
  double _viewHeight = 100;

  @override
  void initState() {
    widget.controller.addListener(() => setState(() {}));
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    double offset = widget.controller.offset / (widget.controller?.position?.maxScrollExtent ?? 1);
    return LayoutBuilder(
      builder: (_, BoxConstraints constraints) {
        _viewHeight = constraints.maxHeight;
        return Stack(children: <Widget>[
          Align(
            alignment: Alignment(1, 1),
            child: Container(color: Colors.red, width: widget.width, height: double.infinity),
          ),
          Align(
            alignment: Alignment(1, -1 + offset * 2),
            child: GestureDetector(
                onVerticalDragUpdate: _handleVerticalDrag, child: Container(width: widget.width, height: 40, color: Colors.green)),
          )
        ]);
      },
    );
  }

  void _handleVerticalDrag(DragUpdateDetails details) {
    double pos = widget.controller.offset;
    double pxRatio = widget.controller.position.maxScrollExtent / _viewHeight;
    widget.controller.jumpTo((pos + details.delta.dy * pxRatio).clamp(0, widget.controller.position.maxScrollExtent));
    //print(pos + details.delta.dx * pxRatio);
  }
}