Scrollbar not draggable for web.

For flutter web, should it be possible to drag the scrollbar? When I mouse-down on it, the labelText is shown, but the tab cannot be dragged.

The code works as expected in the Android emulator (can drag the scrollbar).

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

void main() {
runApp(MyApp());
}

class MyApp extends StatelessWidget {
MyApp({Key key}) : super(key: key);

@override
Widget build(BuildContext context) {

final scrollController = ScrollController();
Widget child = ListView.builder(
physics: ScrollPhysics(),
controller: scrollController,
itemCount: 500,
itemBuilder: rowContent,
);

if (true) {
// Add scrollbar
child = DraggableScrollbar.arrows(
controller: scrollController,
alwaysVisibleScrollThumb: true, //alwaysVisibleScrollThumb,
labelTextBuilder: (double offset) => Text("${offset ~/ 100}"),
child: child,
);
}

return MaterialApp(
debugShowCheckedModeBanner: false,
title: 'MyApp',
home: Container(
color: Colors.blue,
child: child));
}
}

final textStyle = TextStyle(
color: Colors.black,
decoration: TextDecoration.none,
fontSize: 12,
fontStyle: FontStyle.normal);

Widget rowContent(BuildContext context, int i, [int j = 0]) {

return Text(
"$i: $j: abcdef abcdef abcdef abcdef abcdef abcdef abcdef abcdef abcdef abcdef abcdef abcdef",
style: textStyle);
}
```