iOS 15 RC UIWindow makeKeyAndVisible will have black screen appeared
This has continued to be an issue in iOS 15.
I've found that calling makeKeyAndVisible on a window that is already the key window causes the flicker.
To work around this, make sure the window is not visible or not the key window before calling makeKeyAndVisible:
if self.myWindow?.isKeyWindow == false || self.myWindow?.isHidden == true {
self.myWindow?.makeKeyAndVisible()
}