Flutter for Web example background

background.dart

import 'package:flutter_web/material.dart';

class Background extends AnimatedWidget {

  Background({ Key key, @required this.image, @required this.listenable }) 
    : super(key: key, listenable: listenable);

  final AssetImage image;
  final ScrollController listenable;
  
  @override
  Widget build(BuildContext context) {

    double offset = listenable.hasClients ? listenable.offset : 0;
    ScrollPosition position = listenable.hasClients ? listenable.position : null;
    double extent = (position == null) ? 1 : position.maxScrollExtent * 1.2;
    double align = (offset / extent);

    return Container(
      constraints: BoxConstraints.expand(),
      child: Image(
        image: image,
        alignment: Alignment(0, align),
        fit: BoxFit.cover
      )
    );
  }
}