Listview jumps when scrolling from bottom to top

main.dart

import 'package:flutter/material.dart';
import 'package:rxdart/rxdart.dart';

// Example: https://gfycat.com/clumsyslowamericanratsnake

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


class MyBloc {
  BehaviorSubject<String> _someData = BehaviorSubject(seedValue: "Hello");
  Stream<String> get someData => _someData.stream;
}

var bloc = MyBloc();

class MyWidget extends StatelessWidget {
  final String label;
  const MyWidget({Key key, @required this.label}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return StreamBuilder<String>(
      stream: bloc.someData,
      builder: (context, snapshot) {
        return snapshot.hasData
          ? Text("${snapshot.data} - $label")
          : CircularProgressIndicator(); // Or some other tall-ish widget
      }
    );
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text("Test"),
        ),
        body: Center(
          child: ListView.builder(
            itemCount: 50,
            itemBuilder: (ctx, idx) {
              return Padding(
                padding: const EdgeInsets.all(12.0),
                child: MyWidget(label: idx.toString()),
              );
            },
          ),
        ),
      ),
    );
  }
}

pubspec.yaml

name: streambuilder_list_bounce
description: A new Flutter project.
version: 1.0.0+1

environment:
  sdk: ">=2.1.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter
  cupertino_icons: ^0.1.2
  rxdart: ^0.20.0

dev_dependencies:
  flutter_test:
    sdk: flutter


flutter:
  uses-material-design: true