QuickScrollbar class

A quick scrollbar not only indicates which portion of a Scrollable widget is actually visible, but also control the scroll position of Scrollable widget by ScrollController.

Quick scrollbar support the drag gesture to scroll the Scrollable widget quickly. for example:

  QuickScrollbar(
     child: ListView.builder(
        itemCount: 1000,
        // Specifying an [itemExtent] or [prototypeItem]  is more efficient
        // than letting the children determine their own extent when use QuickScrollbar.
        //itemExtent: 56,
        prototypeItem: const ListTile(title: Text("1")),
        itemBuilder: (ctx, index) => ListTile(
          title: Text("$index"),
          onTap: () => debugPrint('$index'),
        ),
      ),
    );
```dart