Fix various layout issues with BottomSheetViewController (stripe#3298)

## Summary
* The BlurView should present and dismiss with the sheet and cover the
unsafe area at the bottom.
* The presenter used to use a small background view at the bottom to
cover the gap between the keyboard and the sheet. Instead, the
BottomSheetViewController should extend to this area.
* When avoiding the keyboard, we were using a generic easeIn curve, so
there were sometimes gaps between the sheet and the keyboard. Use the
animation curve provided by the system instead.
* I'm not sure why the presentation controller was registering for
keyboard events — it makes more sense for the inner scrollview to avoid
the keyboard instead.
* Dismiss the BlurView when entering a 3DS2 challenge.
* Fix the keyboard calculation code on iPad. The FormSheet presenter
moves us around after the keyboard presents, so we need to calculate how
much of the keyboard to avoid based on the position *after* the
presenter moves us.

Before and after screenshots:
![CleanShot 2024-02-15 at 10 16
50](https://github.com/stripe/stripe-ios/assets/52758633/023653ee-799b-4f99-b8ab-a801808b327e)
![CleanShot 2024-02-15 at 10 17
13](https://github.com/stripe/stripe-ios/assets/52758633/597be5c1-7e4f-4158-9ed9-489ea99ebbf4)


## Motivation
Started by trying to fix the BlurView dismiss animation, then
hal-replacing-a-lightbulb.gif from there

## Testing
Existing snapshot tests and E2E UI tests.

## Changelog
None

StripePaymentSheet/StripePaymentSheet/Source/PaymentSheet/BottomSheet/BottomSheetPresentationController.swift

@@ -141,34 +141,20 @@ extension BottomSheetPresentationController {
         containerView.addSubview(presentedView)
 
         // We'll use this constraint to handle the keyboard
-        let bottomAnchor = presentedView.bottomAnchor.constraint(equalTo: containerView.safeAreaLayoutGuide.bottomAnchor)
+        let bottomAnchor = presentedView.bottomAnchor.constraint(equalTo: containerView.bottomAnchor)
+        bottomAnchor.priority = .required
         self.bottomAnchor = bottomAnchor
 
-        // Add a view between the bottom of the VC and the bottom of the screen for 2 reasons
-        // 1. The keyboard animation is sometimes erroneous and results in the bottom of the presented view decoupling from the top of the keyboard, exposing the view behind it.
-        // 2. The presented view (BottomSheetVC) does not inherit safeAreaLayoutGuide.bottom
-        let coverUpBottomView = UIView()
-        containerView.addSubview(coverUpBottomView)
-        coverUpBottomView.backgroundColor = presentedView.backgroundColor
-        coverUpBottomView.translatesAutoresizingMaskIntoConstraints = false
-
         NSLayoutConstraint.activate([
             presentedView.topAnchor.constraint(greaterThanOrEqualTo: containerView.safeAreaLayoutGuide.topAnchor),
             presentedView.leadingAnchor.constraint(equalTo: containerView.safeAreaLayoutGuide.leadingAnchor),
             presentedView.trailingAnchor.constraint(equalTo: containerView.safeAreaLayoutGuide.trailingAnchor),
             bottomAnchor,
-
-            coverUpBottomView.topAnchor.constraint(equalTo: presentedView.bottomAnchor),
-            coverUpBottomView.leadingAnchor.constraint(equalTo: containerView.safeAreaLayoutGuide.leadingAnchor),
-            coverUpBottomView.trailingAnchor.constraint(equalTo: containerView.safeAreaLayoutGuide.trailingAnchor),
-            coverUpBottomView.bottomAnchor.constraint(equalTo: containerView.bottomAnchor),
         ])
 
         fullHeightConstraint.isActive = forceFullHeight
 
         addRoundedCorners(to: presentedView)
-
-        registerForKeyboardNotifications()
     }
 
     // MARK: - Helpers
@@ -178,39 +164,4 @@ extension BottomSheetPresentationController {
         view.layer.cornerRadius = 12
         view.layer.masksToBounds = true
     }
-
-    private func registerForKeyboardNotifications() {
-        NotificationCenter.default.addObserver(
-            self, selector: #selector(adjustForKeyboard),
-            name: UIResponder.keyboardWillHideNotification, object: nil)
-        NotificationCenter.default.addObserver(
-            self, selector: #selector(adjustForKeyboard),
-            name: UIResponder.keyboardWillChangeFrameNotification, object: nil)
-    }
-
-    @objc
-    private func adjustForKeyboard(notification: Notification) {
-        guard
-            let keyboardScreenEndFrame =
-                (notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue)?
-                .cgRectValue,
-            let containerView = containerView,
-            let bottomAnchor = bottomAnchor
-        else {
-            return
-        }
-
-        let keyboardViewEndFrame = containerView.convert(keyboardScreenEndFrame, from: containerView.window)
-        let keyboardInViewHeight = containerView.bounds.intersection(keyboardViewEndFrame).height - containerView.safeAreaInsets.bottom
-        if notification.name == UIResponder.keyboardWillHideNotification {
-            bottomAnchor.constant = 0
-        } else {
-            bottomAnchor.constant = -keyboardInViewHeight
-        }
-
-        containerView.setNeedsLayout()
-        UIView.animateAlongsideKeyboard(notification) {
-            containerView.layoutIfNeeded()
-        }
-    }
 }

StripePaymentSheet/StripePaymentSheet/Source/PaymentSheet/BottomSheet/UIViewController+BottomSheet.swift

@@ -29,6 +29,8 @@ extension UIViewController {
 
         if presentAsFormSheet {
             viewControllerToPresent.modalPresentationStyle = .formSheet
+            // Don't allow the pull down to dismiss gesture, it's too easy to trigger accidentally while scrolling
+            viewControllerToPresent.isModalInPresentation = true
             if let vc = viewControllerToPresent as? BottomSheetViewController {
                 viewControllerToPresent.presentationController?.delegate = vc
             }

StripePaymentSheet/StripePaymentSheet/Source/PaymentSheet/PaymentSheet.swift

@@ -293,7 +293,7 @@ extension PaymentSheet: PaymentSheetViewControllerDelegate {
     func paymentSheetViewControllerFinishedOnPay(_ paymentSheetViewController: PaymentSheetViewController,
                                                  completion: (() -> Void)? = nil) {
         self.bottomSheetViewController.transitionSpinnerToComplete(animated: true) {
-            self.bottomSheetViewController.removeBlurEffect(animated: true, completion: completion)
+            completion?()
         }
     }