title: Flutter remove the space TabBar and SliverAppBar
body: <p>I want to implement a float AppBar with a pinned TabBar at the bottom. The following is the test code (<a href="https://dartpad.dev/fc70c7d8737b5a606517c9af99d90d02" rel="nofollow noreferrer">dartPad</a>):</p>
  <pre><code>Widget build(BuildContext context) {
  return Scaffold(
    body: NestedScrollView(
      floatHeaderSlivers: true,
      headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
        return &lt;Widget&gt;[
          SliverAppBar(
            backgroundColor: Colors.yellow,
            title: Text(
              &quot;WhatsApp type sliver appbar&quot;,
            ),
            centerTitle: true,
            pinned: true,
            floating: true,
            bottom: PreferredSize(
              preferredSize: Size.fromHeight(kToolbarHeight),
              child: Container(
                color: Colors.orange,
                alignment: Alignment.topLeft,
                child: TabBar(
                    isScrollable: true,
                    indicatorColor: Colors.white,
                    indicatorSize: TabBarIndicatorSize.label,
                    controller: _tabController,
                    labelPadding: EdgeInsets.only(
                        top: 13, bottom: 13, left: 16, right: 16),
                    tabs: [
                      Text(
                        &quot;TAB A&quot;,
                      ),
                      Text(
                        &quot;TAB B&quot;,
                      ),
                    ]),
              ),
            ),
          ),
        ];
      },
      body: TabBarView(
        controller: _tabController,
        children: [
          TabA(),
          const Center(
            child: Text('Display Tab 2',
                style: TextStyle(fontSize: 16, fontWeight: FontWeight.bold)),
          ),
        ],
      ),
    ),
  );
  </code></pre>
  <p>}</p>
  <p>I find that it will have a top-padding on the TabBar when scrolling down. Is there any way to remove that space? I have set the SliverAppBar's toolbarheight, but that space will keep there even I lower the height.</p>
  <p>scroll up (show appbar): <a href="https://i.stack.imgur.com/ZRVt7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZRVt7.png" alt="enter image description here" /></a></p>
  <p>scroll down (hide appbar, the top yellow is not hidden): <a href="https://i.stack.imgur.com/kJcsK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kJcsK.png" alt="enter image description here" /></a></p>