ListView with shadow at bottom if overflows

mail.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        brightness: Brightness.dark,
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {

  ScrollController _controller = ScrollController();

  bool showShadow = false;

  @override
  void initState() {
    super.initState();
    Future.delayed(Duration(milliseconds: 500), () {
      setState(() {
        showShadow = _controller.position.extentAfter > 0;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.black,
      body: Stack(
        children: [
          Positioned.fill(
              child: ListView.builder(
                padding: const EdgeInsets.only(bottom: 50.0),
                controller: _controller,
                itemBuilder: (context, index) {
                  return Text("Item $index");
                },
                itemCount: 100,
              )
          ),
          if(showShadow)
          Positioned(
            bottom: 0,
            left: 0,
            right: 0,
            child: Container(height: 60,decoration: BoxDecoration(
              gradient: LinearGradient(
                colors: [Colors.transparent, Colors.black],
                begin: Alignment.topCenter,
                end:Alignment.bottomCenter,
              )
            ),),
          )
        ],
      ),
    );
  }
}