UIVisualEffectView creating unwanted shadow while presenting new view - swift

Question

In my custom presentation transition I've created a new view controller which will pre presented on top of the current active view controller (see screenshot). Somehow there's a shadow behind the blue view controller and I have no idea where it's coming from. Is there a way to stop getting that shadow?
The project is completely empty and has only 2 empty view controllers.
This is the code I'm using:
class ViewController: UIViewController {
let transitionDelegate = TransitionManager()
override func viewDidLoad() {
super.viewDidLoad()
view.backgroundColor = .yellowColor()
let button = UIButton(type: .System)
button.frame = CGRectMake(10, 10, 50, 50)
button.addTarget(self, action: "test:", forControlEvents: .TouchUpInside)
button.backgroundColor = UIColor.redColor()
view.addSubview(button)
}
func test(sender: UIButton) {
let destination = UIViewController()
destination.view.backgroundColor = .blueColor()
destination.transitioningDelegate = transitionDelegate
destination.modalPresentationStyle = .Custom
presentViewController(destination, animated: true, completion: nil)
}
}
The code for presenting the view:
class PresentingTransition: NSObject, UIViewControllerAnimatedTransitioning {
func transitionDuration(transitionContext: UIViewControllerContextTransitioning?) -> NSTimeInterval {
return 0.3
}
func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
let presented = transitionContext.viewControllerForKey(UITransitionContextToViewControllerKey)!
let container = transitionContext.containerView()!
let durations = transitionDuration(transitionContext)
presented.view.alpha = 0
container.addSubview(presented.view)
UIView.animateWithDuration(durations, animations: { presented.view.alpha = 1 }) { transitionContext.completeTransition($0) }
}
}
The code for handling the presenting view controller:
class PresentationController: UIPresentationController {
var background: UIView!
override init(presentedViewController: UIViewController, presentingViewController: UIViewController) {
super.init(presentedViewController: presentedViewController, presentingViewController: presentingViewController)
prepareBackground()
}
func prepareBackground() {
self.background = UIView(frame: presentingViewController.view.bounds)
let blur = UIVisualEffectView(effect: UIBlurEffect(style: .Light))
blur.frame = background.bounds
blur.autoresizingMask = [.FlexibleHeight, .FlexibleWidth]
background.addSubview(blur)
let tapRecognizer = UITapGestureRecognizer(target: self, action: "backgroundTapped:")
background.addGestureRecognizer(tapRecognizer)
}
func backgroundTapped(tapRecognizer: UITapGestureRecognizer) {
presentingViewController.dismissViewControllerAnimated(true, completion: nil)
}
override func presentationTransitionWillBegin() {
let container = containerView!
background.frame = container.bounds
background.alpha = 0.0
container.insertSubview(background, atIndex: 0)
presentedViewController.transitionCoordinator()?.animateAlongsideTransition({ _ in self.background.alpha = 1.0 }, completion: nil)
}
override func dismissalTransitionWillBegin() {
presentedViewController.transitionCoordinator()?.animateAlongsideTransition({ _ in self.background.alpha = 0.0 }, completion: nil)
}
override func frameOfPresentedViewInContainerView() -> CGRect {
return containerView!.bounds.insetBy(dx: 100, dy: 100)
}
override func containerViewWillLayoutSubviews() {
background.frame = containerView!.bounds
presentedView()!.frame = frameOfPresentedViewInContainerView()
}
}