Fix PayNow animation jank (stripe#2934)

## Summary
When the PaymentSheetViewController appears, it refreshes its UI,
including the ConfirmButton's animation state. A race condition arises
when the PaymentSheetViewController and the completion of an action
simultaneously update the UI, resulting in occasional animation
glitches. These occur when we start new animations before the
PaymentSheet animations have completed. To resolve this issue, we delay
the action's evolution to succeeded/cancelled until 0.2 seconds after
the dismissal of the PollingViewController. This allows sufficient time
for the animations to finish, preventing conflicts.

## Motivation
- Polish

## Testing
- Manually ran it 25 times and no longer observed the animation jank

## Changelog
N/A

StripePaymentSheet/StripePaymentSheet/Source/PaymentSheet/PaymentSheetFlowController.swift

@@ -469,8 +469,8 @@ class AuthenticationContext: NSObject, PaymentSheetAuthenticationContext {
         presentingViewController.present(pollingVC, animated: true, completion: nil)
     }
 
-    func dismiss(_ authenticationViewController: UIViewController) {
-        authenticationViewController.dismiss(animated: true, completion: nil)
+    func dismiss(_ authenticationViewController: UIViewController, completion: (() -> Void)?) {
+        authenticationViewController.dismiss(animated: true, completion: completion)
     }
 
     let presentingViewController: UIViewController

StripePaymentSheet/StripePaymentSheet/Source/PaymentSheet/ViewControllers/BottomSheetViewController.swift

@@ -285,11 +285,12 @@ extension BottomSheetViewController: PaymentSheetAuthenticationContext {
         pushContentViewController(pollingVC)
     }
 
-    func dismiss(_ authenticationViewController: UIViewController) {
+    func dismiss(_ authenticationViewController: UIViewController, completion: (() -> Void)?) {
         guard contentViewController is BottomSheet3DS2ViewController || contentViewController is PollingViewController else {
             return
         }
         _ = popContentViewController()
+        completion?()
     }
 }
 

StripePaymentSheet/StripePaymentSheet/Source/PaymentSheet/ViewControllers/PollingViewController.swift

@@ -229,14 +229,18 @@ class PollingViewController: UIViewController {
     // MARK: Handlers
 
     @objc func didTapCancel() {
-        currentAction.complete(with: .canceled, error: nil)
-        dismiss()
+        dismiss {
+            // Wait a short amount of time before completing the action to ensure smooth animations
+            DispatchQueue.main.asyncAfter(deadline: .now() + 0.2) {
+                self.currentAction.complete(with: .canceled, error: nil)
+            }
+        }
     }
 
-    private func dismiss() {
+    private func dismiss(completion: (() -> Void)? = nil) {
         if let authContext = currentAction.authenticationContext as? PaymentSheetAuthenticationContext {
             authContext.authenticationContextWillDismiss?(self)
-            authContext.dismiss(self)
+            authContext.dismiss(self, completion: completion)
         }
 
         oneSecondTimer?.invalidate()
@@ -344,8 +348,12 @@ extension PollingViewController: IntentStatusPollerDelegate {
         if paymentIntent.status == .succeeded {
             setErrorStateWorkItem.cancel() // cancel the error work item incase it was scheduled
             currentAction.paymentIntent = paymentIntent // update the local copy of the intent with the latest from the server
-            currentAction.complete(with: .succeeded, error: nil)
-            dismiss()
+            dismiss {
+                // Wait a short amount of time before completing the action to ensure smooth animations
+                DispatchQueue.main.asyncAfter(deadline: .now() + 0.2) {
+                    self.currentAction.complete(with: .succeeded, error: nil)
+                }
+            }
         } else if paymentIntent.status != .requiresAction {
             // an error occured to take the intent out of requires action
             // update polling state to indicate that we have encountered an error

StripePaymentSheet/StripePaymentSheetTests/PaymentSheet/PaymentSheetLPMConfirmFlowTests.swift

@@ -446,8 +446,8 @@ extension PaymentSheet_LPM_ConfirmFlowTests: PaymentSheetAuthenticationContext {
         // no-op
     }
 
-    func dismiss(_ authenticationViewController: UIViewController) {
-        // no-op
+    func dismiss(_ authenticationViewController: UIViewController, completion: (() -> Void)?) {
+        completion?()
     }
 
     func presentPollingVCForAction(action: STPPaymentHandlerActionParams, type: STPPaymentMethodType, safariViewController: SFSafariViewController?) {

StripePayments/StripePayments/Source/PaymentHandler/STPPaymentHandler.swift

@@ -2331,7 +2331,7 @@ extension STPPaymentHandler {
         if let paymentSheet = currentAction.authenticationContext
             .authenticationPresentingViewController() as? PaymentSheetAuthenticationContext
         {
-            paymentSheet.dismiss(challengeViewController)
+            paymentSheet.dismiss(challengeViewController, completion: nil)
         } else {
             challengeViewController.dismiss(animated: true, completion: nil)
         }
@@ -2349,7 +2349,7 @@ extension STPPaymentHandler {
 /// Internal authentication context for PaymentSheet magic
 @_spi(STP) public protocol PaymentSheetAuthenticationContext: STPAuthenticationContext {
     func present(_ authenticationViewController: UIViewController, completion: @escaping () -> Void)
-    func dismiss(_ authenticationViewController: UIViewController)
+    func dismiss(_ authenticationViewController: UIViewController, completion: (() -> Void)?)
     func presentPollingVCForAction(action: STPPaymentHandlerActionParams, type: STPPaymentMethodType, safariViewController: SFSafariViewController?)
 }