title: Make SliverAppBar scrollable when TabBarView child is scrolled
body: <p>I have home screen with bottom navigation consist of two items which all have ListView inside with infinite list and I want SliverAppBar to be scrollable when user scrolls in one of the list.
  Here is what I have so far</p>
  
  <pre><code>class HomeScreen extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return DefaultTabController(
          length: 2,
          child: new Scaffold(
            body: CustomScrollView(
              slivers: &lt;Widget&gt;[
                SliverAppBar(
                  snap: false,
                  floating: false,
                  pinned: true,
                  expandedHeight: 160.0,
                  flexibleSpace: FlexibleSpaceBar(
                    title: Text('Title'),
                  ),
                ),
                SliverFillRemaining(
                  child: TabBarView(
                    children: &lt;Widget&gt;[Items(), Activities()], //THESE HAVE LIST VIEW IN EACH 
                  ),
                )
              ],
            ),
          ));
    }
  }
  </code></pre>
  
  <p>And here is code of one of TabBarView children.</p>
  
  <pre><code>class Items extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      List&lt;Widget&gt; itemsWidgets = List.from(getItemsList()
          .map((Item item) =&gt; createItemWidget(context, item)));
  
      return Scaffold(
        body: Center(
          child: ListView(
            children: itemsWidgets,
          ),
        ),
      );
    }
  
    ListTile createItemWidget(BuildContext context, Item item) {
      return new ListTile(
        title: Text(item.sender.name),
        subtitle: Text('10:30 am'),
      );
    }
  }
  </code></pre>
  
  <p>How can SilverAppBar be scrollable when user scrolls in one of the list? Any help/suggestion will be appreciated.</p>