Cherry-pick #19961 into hotfix 21.5.1 (#19972)

Author: mokagio

RELEASE-NOTES.txt

@@ -2,6 +2,10 @@
 -----
 * [*] Fix a layout issue impacting the "No media matching your search" empty state message of the Media Picker screen.  [#19820]
 
+21.5.1
+-----
+* [*] [Jetpack-only] Fixed a bug where the Login flow was restarting every time the app enters the foreground. [#19961]
+
 21.5
 -----
 * [***] [internal] A significant refactor to the app’s architecture was made to allow for the new simplified UI. Regression testing on the app’s main flows is needed. [#19817]

WordPress/Classes/System/WindowManager.swift

@@ -21,6 +21,12 @@ class WindowManager: NSObject {
     ///
     private(set) var isShowingFullscreenSignIn = false
 
+    /// The root view controller for the window.
+    ///
+    var rootViewController: UIViewController? {
+        return window.rootViewController
+    }
+
     init(window: UIWindow) {
         self.window = window
     }

WordPress/Jetpack/Classes/System/JetpackWindowManager.swift

@@ -139,11 +139,41 @@ private extension JetpackWindowManager {
             !hasFailedExportAttempts,
             let schemeUrl = URL(string: "\(AppScheme.wordpressMigrationV1.rawValue)\(WordPressExportRoute().path.removingPrefix("/"))")
         else {
-            showSignInUI()
+            performSafeRootNavigation { [weak self] in
+                self?.showSignInUI()
+            }
             return
         }
 
         /// WordPress is a compatible version for migrations, but needs to be loaded to prepare the data
-        showLoadWordPressUI(schemeUrl: schemeUrl)
+        performSafeRootNavigation { [weak self] in
+            self?.showLoadWordPressUI(schemeUrl: schemeUrl)
+        }
+    }
+
+    /// This method takes care of preventing screens being abruptly replaced.
+    ///
+    /// Since the import method is called whenever the app is foregrounded, we want to make sure that
+    /// any root view controller replacements only happen where it is "allowed":
+    ///
+    ///   1. When there's no root view controller yet, or
+    ///   2. When the Load WordPress screen is shown.
+    ///
+    /// Note: We should remove this method when the migration phase is concluded and we no longer need
+    /// to perfom the migration.
+    ///
+    /// - Parameter navigationClosure: The closure containing logic that eventually calls the `show` method.
+    func performSafeRootNavigation(with navigationClosure: @escaping () -> Void) {
+        switch rootViewController {
+        case .none:
+            // we can perform the navigation directly when there's no root view controller yet.
+            navigationClosure()
+        case .some(let viewController) where viewController is MigrationLoadWordPressViewController:
+            // allow the Load WordPress view to be replaced in case the migration process fails.
+            viewController.dismiss(animated: true, completion: navigationClosure)
+        default:
+            // do nothing when another root view controller is already displayed.
+            break
+        }
     }
 }