title: Flutter: How to get accessibility focus to custom appbar
body: <p>If you use the default Material scaffold with the default appbar, and you open your app using TalkBack, the accessibility focus moves automatically to the first item in the appbar (usually, the "Back" button). </p>
  
  <p>I would like my appbar to be taller than the default, so I've created a custom one and pass it to the scaffold:</p>
  
  <pre><code>class CustomAppbar extends StatelessWidget implements PreferredSizeWidget {
    @override
    Widget build(BuildContext context) {
      return Container(
        height: 120,
        child: Semantics(
          focused: true,
          child: RaisedButton(
            child: Text('Test'),
            onPressed: () {},
          ),
        ),
      );
    }
  
    @override
    Size get preferredSize =&gt; Size.fromHeight(120);
  }
  
  </code></pre>
  
  <pre><code>Widget build(BuildContext context) {
      return Scaffold(
        appBar: CustomAppbar(),
        body: SafeArea(
          child: Stack(children: &lt;Widget&gt;[
            Column(children: &lt;Widget&gt;[
              _buildContent(),
            ]),
          ]),
        ),
      );
    }
  </code></pre>
  
  <p>The problem is, <strong>if I don't use the default AppBar, I can't move the focus to the first button</strong>, even if I wrap it with Semantics. </p>
  
  <p>How is the default Scaffold handling the automatic focus? I've searched in the code and couldn't find it. </p>