Gallery dismiss interaction fixes

Packages/GalleryVC/Sources/GalleryVC/GalleryDismissInteraction.swift

@@ -20,6 +20,8 @@ class GalleryDismissInteraction: NSObject {
     private(set) var dismissVelocity: CGPoint?
     private(set) var dismissTranslation: CGPoint?
     
+    private var cancelAnimator: UIViewPropertyAnimator?
+    
     init(viewController: GalleryViewController) {
         self.viewController = viewController
         super.init()
@@ -38,6 +40,8 @@ class GalleryDismissInteraction: NSObject {
             content = viewController.currentItemViewController.takeContent()
             content!.view.translatesAutoresizingMaskIntoConstraints = true
             content!.view.frame = origContentFrameInGallery!
+            // Make sure the context remains behind the controls
+            content!.view.layer.zPosition = -1000
             viewController.view.addSubview(content!.view)
             
             origControlsVisible = viewController.currentItemViewController.controlsVisible
@@ -59,20 +63,27 @@ class GalleryDismissInteraction: NSObject {
             if translationMagnitude < 150 && velocityMagnitude < 500 {
                 isActive = false
                 
+                cancelAnimator?.stopAnimation(true)
+                
                 let spring = UISpringTimingParameters(mass: 1, stiffness: 439, damping: 42, initialVelocity: .zero)
-                let animator = UIViewPropertyAnimator(duration: 0.2, timingParameters: spring)
-                animator.addAnimations {
+                cancelAnimator = UIViewPropertyAnimator(duration: 0.2, timingParameters: spring)
+                cancelAnimator!.addAnimations {
                     self.content!.view.frame = self.origContentFrameInGallery!
                     self.viewController.currentItemViewController.setControlsVisible(self.origControlsVisible!, animated: false, dueToUserInteraction: false)
                 }
-                animator.addCompletion { _ in
+                cancelAnimator!.addCompletion { _ in
+                    guard !self.isActive else {
+                        // bail in case the animation finishing raced with the user's interaction
+                        return
+                    }
+                    self.content!.view.layer.zPosition = 0
                     self.content!.view.removeFromSuperview()
                     self.viewController.currentItemViewController.addContent()
                     self.content = nil
                     self.origContentFrameInGallery = nil
                     self.origControlsVisible = nil
                 }
-                animator.startAnimation()
+                cancelAnimator!.startAnimation()
 
             } else {
                 dismissVelocity = velocity