Show jump to present toast if necessary when scene re-appears

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -125,6 +125,8 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                cell.update()
            }
         }
+        
+        NotificationCenter.default.addObserver(self, selector: #selector(sceneWillEnterForeground), name: UIScene.willEnterForegroundNotification, object: nil)
     }
     
     // separate method because InstanceTimelineViewController needs to be able to customize it
@@ -242,6 +244,20 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
 //        }
 //    }
     
+    @objc private func sceneWillEnterForeground(_ notification: Foundation.Notification) {
+        guard let scene = notification.object as? UIScene,
+              // view.window is nil when the VC is not on screen
+              view.window?.windowScene == scene else {
+            return
+        }
+        Task {
+            if case .idle = controller.state,
+               let presentItems = try? await loadInitial() {
+                insertPresentItemsIfNecessary(presentItems)
+            }
+        }
+    }
+    
     @objc func refresh() {
         Task {
             if case .notLoadedInitial = controller.state {
@@ -249,24 +265,48 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             } else {
                 // I'm not sure whether this should move into TimelineLikeController/TimelineLikeCollectionViewController
                 let (_, presentItems) = await (controller.loadNewer(), try? loadInitial())
-                if let presentItems,
+                if let presentItems {
-                   case .status(id: let id, state: _) = dataSource.snapshot().itemIdentifiers(inSection: .statuses).first {
+                    insertPresentItemsIfNecessary(presentItems)
-                    // if there's no overlap between presentItems and the existing items in the data source, prompt the user to scroll to present
+                }
-                    if !presentItems.contains(id) {
+            }
-                        var snapshot = self.dataSource.snapshot()
+            #if !targetEnvironment(macCatalyst)
-                        let currentItems = snapshot.itemIdentifiers(inSection: .statuses)
+            collectionView.refreshControl?.endRefreshing()
-                        guard let item = currentItems.first,
+            #endif
-                              case .status(id: id, state: _) = item else {
+        }
-                            return
     }
     
-                        // remove any existing gap if there is one
+    private func insertPresentItemsIfNecessary(_ presentItems: [String]) {
+        var snapshot = dataSource.snapshot()
+        let currentItems = snapshot.itemIdentifiers(inSection: .statuses)
+        if case .status(id: let firstID, state: _) = currentItems.first,
+           // if there's no overlap between presentItems and the existing items in the data source, insert the present items and prompt the user
+           !presentItems.contains(firstID) {
+            let applySnapshotBeforeScrolling: Bool
+            
+            // remove any existing gap, if there is one
             if let index = currentItems.lastIndex(of: .gap) {
                 snapshot.deleteItems(Array(currentItems[index...]))
+                
+                let statusesSection = snapshot.indexOfSection(.statuses)!
+                if collectionView.indexPathsForVisibleItems.contains(IndexPath(row: index, section: statusesSection)) {
+                    // the gap cell is on screen
+                    applySnapshotBeforeScrolling = false
+                } else if let topMostVisibleCell = collectionView.indexPathsForVisibleItems.first(where: { $0.section == statusesSection }),
+                          index < topMostVisibleCell.row {
+                    // the gap cell is above the top, so applying the snapshot would remove the currently-viewed statuses
+                    applySnapshotBeforeScrolling = false
+                } else {
+                    // the gap cell is below the bottom of the screen
+                    applySnapshotBeforeScrolling = true
                 }
-                        snapshot.insertItems([.gap], beforeItem: item)
+            } else {
+                // there is no existing gap
+                applySnapshotBeforeScrolling = true
+            }
+            snapshot.insertItems([.gap], beforeItem: currentItems.first!)
             snapshot.insertItems(presentItems.map { .status(id: $0, state: .unknown) }, beforeItem: .gap)
             
+            if applySnapshotBeforeScrolling {
                 // use a snapshot of the collection view to hide the flicker as the content offset changes and then changes back
                 let snapshotView = collectionView.snapshotView(afterScreenUpdates: false)!
                 snapshotView.layer.zPosition = 1000
@@ -278,6 +318,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                     self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
                     snapshotView.removeFromSuperview()
                 }
+            }
             
             var config = ToastConfiguration(title: "Jump to present")
             config.edge = .top
@@ -286,17 +327,15 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             config.action = { [unowned self] toast in
                 toast.dismissToast(animated: true)
                 
+                if !applySnapshotBeforeScrolling {
+                    self.dataSource.apply(snapshot, animatingDifferences: false)
+                }
+                
                 self.collectionView.scrollToTop()
             }
             self.showToast(configuration: config, animated: true)
         }
     }
-            }
-            #if !targetEnvironment(macCatalyst)
-            collectionView.refreshControl?.endRefreshing()
-            #endif
-        }
-    }
     
 }