title: Header sliver not floating over the list in a NestedScrollView
body: <p>I have a Material Design screen where I use a <code>RefreshIndicator</code> for pull-to-refresh effect. However, I need the <code>RefreshIndicator</code> to appear below the <code>SliverAppBar</code>; due to that, I can't just leave the <code>SliverAppBar</code> inside the list's <code>CustomScrollView</code> and I need to give it a header place in a <code>NestedScrollView</code>.</p>
  <p>The problem comes because I can't seem to <strong>synchronize the general scroll of the list and the header</strong>. The list scrolls fine, but the header does not. I want it to float and snap upon scroll up/down.</p>
  <pre class="lang-dart prettyprint-override"><code>
  class Sample extends StatefulWidget {
    @override
    _SampleState createState() =&gt; _SampleState();
  }
  
  class _SampleState extends State&lt;Sample&gt; {
    final ScrollController _scrollController = new ScrollController();
  
    @override
    Widget build(BuildContext context) {
      return Scaffold(
        body: NestedScrollView(
          headerSliverBuilder: (context, _) =&gt; [
            SliverOverlapAbsorber(
              handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
              sliver: SliverAppBar(
                floating: true,
                snap: true,
                title: Text('Title'),
              ),
            )
          ],
          body: Builder(
            builder: (context) =&gt; RefreshIndicator(
              onRefresh: () { /* do stuff */ },
              child: CustomScrollView(
                controller: _scrollController, // needed for an infinite scroll items loading effect
                slivers: &lt;Widget&gt;[
                  SliverOverlapInjector(
                    handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context)
                  )
                  // more slivers
                ],
              ),
            )
          ),
        ),
      );
    }
  }
  </code></pre>
  <p>I guess I am missing something regarding the injecting/absorbing of the header sliver overlap.</p>
  <p><strong>How can I make the <code>SliverAppBar</code> and the general <code>CustomScrollView</code> scroll synchronously?</strong></p>