title: React Navigation get stack header height
body: <p>How do i get the height of the StackNavigator header programmatically?</p>
  
  <p>I need to detect if component A's position is within component B when pan gesture (drag) on A is released. However, the position from <code>onPanResponderRelease</code>'s <code>gesture.moveY</code> is measured with respect to the entire screen, whereas the position returned from A's <code>onLayout</code>is measured with respect to the parent view. So i'd need to know the parent view's current height to reconcile the differences.</p>