Prune offscreen rows on new timeline

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -160,7 +160,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
     override func viewDidDisappear(_ animated: Bool) {
         super.viewDidDisappear(animated)
         
-        // TODO: prune offscreen rows
+        pruneOffscreenRows()
     }
     
     private func removeTimelineDescriptionCell() {
@@ -170,6 +170,25 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         isShowingTimelineDescription = false
     }
     
+    private func pruneOffscreenRows() {
+        guard let lastVisibleIndexPath = collectionView.indexPathsForVisibleItems.last else {
+            return
+        }
+        var snapshot = dataSource.snapshot()
+        let items = snapshot.itemIdentifiers(inSection: .statuses)
+        let pageSize = 20
+        let numberOfPagesToPrune = (items.count - lastVisibleIndexPath.row - 1) / pageSize
+        if numberOfPagesToPrune > 0 {
+            let itemsToRemove = Array(items.suffix(numberOfPagesToPrune * pageSize))
+            snapshot.deleteItems(itemsToRemove)
+        }
+        dataSource.apply(snapshot, animatingDifferences: false)
+        
+        if case .status(id: let id, state: _) = snapshot.itemIdentifiers(inSection: .statuses).last {
+            older = .before(id: id, count: nil)
+        }
+    }
+    
     @objc func refresh() {
         Task {
             await controller.loadNewer()