title: UINavigationBar with Large Titles - how to find extra height in iOS 11
body: <p>When using prefersLargeTitles for a <code>UINavigationController</code>'s <code>UINavigationBar</code> in <code>iOS 11</code>, the nav bar increases height. The increase is from 44 to 96 on the iPhones I have checked, but I think those numbers can change per device (or at least we need to code as if they can). </p>
  
  <p>I want to programmatically find the 'extra' height - the height of the large titles area that is added beneath the traditional <code>UINavigationBar</code> when a large title is displayed. I can easily find the entire height of the bar with the large title displayed, but is there a way to programmatically find the height of the large title portion of the bar alone (without any hardcoding)? </p>
  
  <p>The reason I need this is that there are times that I want to programmatically scroll to the top of a UITableView, pulling down the large title (which has scrolled up under the "regular height" nav bar) so that it is showing, and the content offset I need is the extra height of the nav bar. I could use the total height of the navigation bar, but this would pull the UITableView down too far. To do this now, I need to hardcode as below:</p>
  
  <pre><code>[self.tableView setContentOffset:CGPointMake(0, -52) animated:NO];
  </code></pre>