Invalidate user activities when collapsed and returning to Feeds scene

Shared/Activity/ActivityManager.swift

@@ -29,6 +29,13 @@ class ActivityManager {
 		NotificationCenter.default.addObserver(self, selector: #selector(feedIconDidBecomeAvailable(_:)), name: .FeedIconDidBecomeAvailable, object: nil)
 	}
 	
+	func invalidateCurrentActivities() {
+		readingActivity?.invalidate()
+		readingActivity = nil
+		selectingActivity?.invalidate()
+		selectingActivity = nil
+	}
+	
 	func selectingToday() {
 		let title = NSLocalizedString("See articles for Today", comment: "Today")
 		selectingActivity = makeSelectingActivity(type: ActivityType.selectToday, title: title, identifier: "smartfeed.today")

iOS/SceneCoordinator.swift

@@ -253,6 +253,7 @@ class SceneCoordinator: NSObject, UndoableCommandRunner, UnreadCountProvider {
 		rootSplitViewController.delegate = self
 		
 		masterNavigationController = (rootSplitViewController.viewControllers.first as! UINavigationController)
+		masterNavigationController.delegate = self
 		
 		masterFeedViewController = UIStoryboard.main.instantiateController(ofType: MasterFeedViewController.self)
 		masterFeedViewController.coordinator = self
@@ -784,6 +785,16 @@ extension SceneCoordinator: UISplitViewControllerDelegate {
 	
 }
 
+// MARK: UINavigationControllerDelegate
+
+extension SceneCoordinator: UINavigationControllerDelegate {
+	func navigationController(_ navigationController: UINavigationController, didShow viewController: UIViewController, animated: Bool) {
+		if rootSplitViewController.isCollapsed && viewController === masterFeedViewController {
+			activityManager.invalidateCurrentActivities()
+		}
+	}
+}
+
 // MARK: Private
 
 private extension SceneCoordinator {