title: Avoid header being pushed by KeyboardAvoidingView
body: <p>I have a container component in a stack navigator with the header on top, a scrollview containing messages and a input element</p>
  
  <p>When I focus the input the whole window gets pushed up, hiding also the header</p>
  
  <p>How can I avoid this behaviour?</p>
  
  <p>This is the component I'm using:</p>
  
  <pre><code>class Chat extends Component {
    static navigationOptions = ({navigation}) =&gt; {
      return {
        headerTitle: (
          &lt;Header
            user={navigation.getParam('chat').partecipants[0]}
            headerClick={() =&gt; navigation.getParam('openProfile')()}
          /&gt;
        )
      }
    }
  
  .....
  
    render() {
      ...
      return (
    &lt;KeyboardAvoidingView
        style={{flex: 1}}
        behavior={'padding'}
        &gt;
        &lt;ScrollView&gt;
          {messages}
        &lt;/ScrollView&gt;
        &lt;Input sendMessage={(m) =&gt; this.props.sendMessage(chat, m)}/&gt;
      &lt;/KeyboardAvoidingView&gt;
  
      )
    }
  }
  </code></pre>