Fix notifications scrolling to top when refreshing

Tusker/Screens/Notifications/NotificationsCollectionViewController.swift

@@ -355,8 +355,24 @@ extension NotificationsCollectionViewController {
     }
     
     func handlePrependItems(_ timelineItems: [NotificationGroup]) async {
+        let topItem = dataSource.snapshot().itemIdentifiers(inSection: .notifications).first
+        
         // we always replace all, because new items are merged with existing ones
         await handleReplaceAllItems(timelineItems)
+        
+        // preserve the scroll position
+        // todo: this won't work for cmd+r when not at top
+        if let topID = topItem?.group?.notifications.first?.id {
+            // the exact item may have changed, due to merging
+            let newTopGroup = timelineItems.first {
+                $0.notifications.contains {
+                    $0.id == topID
+                }
+            }!
+            if let newTopIndexPath = dataSource.indexPath(for: .group(newTopGroup)) {
+                collectionView.scrollToItem(at: newTopIndexPath, at: .top, animated: false)
+            }
+        }
     }
     
     func loadOlder() async throws -> [NotificationGroup] {

Tusker/Screens/Utilities/TimelineLikeCollectionViewController.swift

@@ -153,6 +153,7 @@ extension TimelineLikeCollectionViewController {
         }
         await apply(snapshot, animatingDifferences: false)
         
+        // todo: this won't work for cmd+r when not at top
         if let first,
            let indexPath = dataSource.indexPath(for: first) {
             // TODO: i can't tell if this actually works or not