Don't dismiss gallery if pan distance/velocity is low

See #520

Packages/GalleryVC/Sources/GalleryVC/GalleryDismissInteraction.swift

@@ -53,12 +53,35 @@ class GalleryDismissInteraction: NSObject {
             let translation = recognizer.translation(in: viewController.view)
             let velocity = recognizer.velocity(in: viewController.view)
             
-            dismissVelocity = velocity
-            dismissTranslation = translation
-            viewController.dismiss(animated: true)
+            let translationMagnitude = sqrt(translation.x.magnitudeSquared + translation.y.magnitudeSquared)
+            let velocityMagnitude = sqrt(velocity.x.magnitudeSquared + velocity.y.magnitudeSquared)
             
-            // don't unset this until after dismiss is called, so that the dismiss animation controller can read it
-            isActive = false
+            if translationMagnitude < 150 && velocityMagnitude < 500 {
+                isActive = false
+                
+                let spring = UISpringTimingParameters(mass: 1, stiffness: 439, damping: 42, initialVelocity: .zero)
+                let animator = UIViewPropertyAnimator(duration: 0.2, timingParameters: spring)
+                animator.addAnimations {
+                    self.content!.view.frame = self.origContentFrameInGallery!
+                    self.viewController.currentItemViewController.setControlsVisible(self.origControlsVisible!, animated: false, dueToUserInteraction: false)
+                }
+                animator.addCompletion { _ in
+                    self.content!.view.removeFromSuperview()
+                    self.viewController.currentItemViewController.addContent()
+                    self.content = nil
+                    self.origContentFrameInGallery = nil
+                    self.origControlsVisible = nil
+                }
+                animator.startAnimation()
+
+            } else {
+                dismissVelocity = velocity
+                dismissTranslation = translation
+                viewController.dismiss(animated: true)
+                
+                // don't unset this until after dismiss is called, so that the dismiss animation controller can read it
+                isActive = false
+            }
             
         default:
             break