title: NSPopover animate contentViewController change and change popover size
body: <p>I am making a NSPopover, and have made it so I can transition between view controllers with a sliding animation using a parent view controller with my view controllers as children. It works well, except before I added this, the popover automatically resized to my view's size, but now, the popover is stuck at a fixed size.</p>
  
  <p>The code for creating the popover:</p>
  
  <pre><code>    self.homeVC = PopoverViewController(nibName: "PopoverViewController", bundle: nil)
      self.loginVC = SignInViewController(nibName: "SignInViewController", bundle: nil)
  
      self.containerView.view.wantsLayer = true
  
      self.containerView.view.frame = self.homeVC!.view.bounds
  
      self.containerView.addChildViewController(self.homeVC!)
  
      self.containerView.view.addSubview(self.homeVC!.view)
  
      popover.contentViewController = self.containerView
  </code></pre>
  
  <p>The code for transitioning the view controllers:</p>
  
  <pre><code>    self.loginVC!.view.frame = self.homeVC!.view.bounds
      self.containerView.addChildViewController(self.loginVC!)
  
      let transition: NSViewControllerTransitionOptions = .SlideLeft
  
      self.containerView.transitionFromViewController(self.homeVC!, toViewController: self.loginVC!, options: transition, completionHandler: { finished in
          self.homeVC!.view.removeFromSuperview()
          self.homeVC!.removeFromParentViewController()
          self.containerView.view.bounds = self.loginVC!.view.bounds
      })
  </code></pre>
  
  <p>Is there anyway that I can make the popover automatically resize to the size it is supposed to be after transitioning?</p>
  
  <p>Thanks in advance.</p>