Flutter: ListView not scrollable, not bouncing

Just add AlwaysScrollableScrollPhysics

ListView(
        physics: const AlwaysScrollableScrollPhysics(),
        children :  [...]
}

To always have the scroll enabled on a ListView you can wrap the original scroll phisics you want with the AlwaysScrollableScrollPhysics class. More details here. If you want you can specify a parent or rely on the default.

Here is your example with the option added:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  ScrollController _controller = new ScrollController();

  @override
  Widget build(BuildContext context) {
    return new ListView(
        physics: const AlwaysScrollableScrollPhysics(), // new
        controller: _controller,
        children: <Widget>[
          new Container(
            height: 40.0,
            color: Colors.blue,
          ),
          new Container(
            height: 40.0,
            color: Colors.red,
          ),
          new Container(
            height: 40.0,
            color: Colors.green,
          ),
        ]
    );
  }
}

if you are facing this problem on Flutter Web then You should wrap listview with ScrollConfiguration. Add PointerDeviceKind.mouse in ScrollConfiguration behavior argument. You can see example.

ScrollConfiguration(
behavior: ScrollConfiguration.of(context).copyWith(dragDevices: {
  PointerDeviceKind.touch,
  PointerDeviceKind.mouse,
},),
child: ListView(
  controller: _controller,
  physics: const AlwaysScrollableScrollPhysics(),
  scrollDirection: Axis.horizontal,
  children: <Widget>[
  
      for (int index = 0; index < showThumbnailList.length; index++) _thumbnail(showThumbnailList[index], index)
   
    // showThumbnailList.map((x) => _thumbnail(x) ).toList()),
  ],
),

),