Weird UIVisualEffectView behavior in multitasking - swift

Question

I have an AccountsViewController which has UIVisualEffectView with blur effect in background (subview at index 0), so that it covers previous controller with blur during transition. But when I then switch to another app and open multitasking menu again to switch back to my app, its blurry UIVisualEffectView seem to be damaged (as shown on the screenshot 1). When I make my app active again, the view gets "repaired" and looks ok again (as shown on the second screenshot)
Some code from my custom transitioning class where I add the blur view
// in the animateTransition.. method
guard let container = transitionContext.containerView() else {
return
}
let blurEffectView: UIVisualEffectView
if container.viewWithTag(101) != nil {
blurEffectView = container.viewWithTag(101)! as! UIVisualEffectView
}
else {
blurEffectView = UIVisualEffectView(frame: container.frame)
blurEffectView.translatesAutoresizingMaskIntoConstraints = false
blurEffectView.tag = 101
container.addSubview(blurEffectView)
}
if self.isPresenting {
// We're presenting a view controller over another
toViewController.view.transform = CGAffineTransformMakeScale(0.7, 0.7)
container.addSubview(toViewController.view)
container.sendSubviewToBack(blurEffectView)
toViewController.view.alpha = 0
UIView.animateWithDuration(self.transitionDuration(transitionContext), delay: 0, usingSpringWithDamping: 0.8, initialSpringVelocity: 0.8, options: [], animations: { () -> Void in
fromViewController.view.transform = CGAffineTransformMakeScale(0.8, 0.8)
toViewController.view.transform = CGAffineTransformMakeScale(1, 1)
blurEffectView.effect = UIBlurEffect(style: .Dark)
toViewController.view.alpha = 1
}, completion: { (completed) -> Void in
self.context?.completeTransition(completed)
})
}
else {
// We're dismissing a view controller
UIView.animateWithDuration(self.transitionDuration(transitionContext), delay: 0, usingSpringWithDamping: 0.8, initialSpringVelocity: 0.8, options: [], animations: { () -> Void in
fromViewController.view.transform = CGAffineTransformMakeScale(0.7, 0.7)
toViewController.view.transform = CGAffineTransformMakeScale(1, 1)
blurEffectView.effect = nil
fromViewController.view.alpha = 0
}, completion: { (completed) -> Void in
fromViewController.view.removeFromSuperview()
self.context?.completeTransition(completed)
})
}

Answer 1

Seems like I've figured it out myself: this issue appears only when there is an UIWindow back (which is black) showing through.
In this question I had a transition animation container with these views:
fromViewController's view (with a scale transform 0.8;0.8)
UIVisualEffectView with .Dark style blur applied to it
toViewController's view
Because of fromViewController's view size (which was smaller than the screen because of scale transform), the black UIWindow view was showing through. And according to my research this causes UIVisualEffectView to produce a glitch described in this question.
The solution is to add a black UIView with the frame of an UIWindow and add it to the bottom of the container's view hierarchy:
let blackView = UIView(frame: container.frame)
blackView.backgroundColor = UIColor.blackColor()
container.insertSubview(blackView, atIndex: 0)
And then this glitch seem to appear no longer.