title: Full-screen UICollectionViewCell issue on iPhone X in landscape
body: <p>In my app, I present a view controller modally, and this view controller contains a <code>UICollectionView</code> that fills the superview. Each cell in the collection view needs to be full-screen - the same size as the view's bounds. To accomplish this, I simply set the <code>itemSize</code> of the <code>UICollectionViewFlowLayout</code> to be the view's bound's size in <code>viewDidLayoutSubviews</code>. </p>
  
  <p>This works really well on all devices and orientations, except iPhone X in landscape. When presenting that screen in that case, it seems the safe area layout margins are coming into play and affecting positioning of the cell. The cell is pushed over from the left edge of the display, and pushed up such that the bottom of the cell is aligned to the vertical center of the home indicator. Inspecting the layout with View Debugger reveals the collection view fills the screen, but the cell's Y position is -10.67 and the X position is 0, even though it is inset from the leading of the collection view. Notably, this is logged in the console:</p>
  
  <pre><code>The behavior of the UICollectionViewFlowLayout is not defined because:
  the item height must be less than the height of the UICollectionView minus the section insets top and bottom values, minus the content insets top and bottom values.
  The relevant UICollectionViewFlowLayout instance is &lt;UICollectionViewFlowLayout: 0x7fa6311253b0&gt;, and it is attached to &lt;UICollectionView: 0x7fa63207e800; frame = (0 0; 812 375); clipsToBounds = YES; autoresize = RM+BM; gestureRecognizers = &lt;NSArray: 0x608000244d70&gt;; layer = &lt;CALayer: 0x60800023ef00&gt;; contentOffset: {-44, 0}; contentSize: {0, 812}; adjustedContentInset: {0, 44, 21, 44}&gt; collection view layout: &lt;UICollectionViewFlowLayout: 0x7fa6311253b0&gt;.
  </code></pre>
  
  <p>What's going on here and how can this be resolved?</p>