Change timeline gap-filling to do a proper job of maintaining the bottom-relative scroll position

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -246,17 +246,9 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             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 firstVisibleIndexPath = collectionView.indexPathsForVisibleItems.min()!
-                let snapshotView = collectionView.snapshotView(afterScreenUpdates: false)!
+                let firstVisibleItem = dataSource.itemIdentifier(for: firstVisibleIndexPath)!
-                snapshotView.layer.zPosition = 1000
+                applySnapshot(snapshot, maintainingBottomRelativeScrollPositionOf: firstVisibleItem)
-                snapshotView.frame = view.bounds
-                view.addSubview(snapshotView)
-                
-                let bottomOffset = collectionView.contentSize.height - collectionView.contentOffset.y
-                self.dataSource.apply(snapshot, animatingDifferences: false) {
-                    self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
-                    snapshotView.removeFromSuperview()
-                }
             }
             
             var config = ToastConfiguration(title: "Jump to present")
@@ -276,6 +268,50 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         }
     }
     
+    // NOTE: this only works when items are being inserted ABOVE the item to maintain
+    private func applySnapshot(_ snapshot: NSDiffableDataSourceSnapshot<Section, Item>, maintainingBottomRelativeScrollPositionOf itemToMaintain: Item) {
+        // 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
+        snapshotView.frame = view.bounds
+        view.addSubview(snapshotView)
+        
+        var firstItemAfterOriginalGapOffsetFromTop: CGFloat = 0
+        if let indexPath = dataSource.indexPath(for: itemToMaintain),
+           let cell = collectionView.cellForItem(at: indexPath) {
+            // subtract top safe area inset b/c scrollToItem at .top aligns the top of the cell to the top of the safe area
+            firstItemAfterOriginalGapOffsetFromTop = cell.convert(.zero, to: view).y - view.safeAreaInsets.top
+        }
+        
+        dataSource.apply(snapshot, animatingDifferences: false) {
+            if let indexPathOfItemAfterOriginalGap = self.dataSource.indexPath(for: itemToMaintain) {
+                // scroll up until we've accumulated enough MEASURED height that we can put the
+                // firstItemAfterOriginalGapCell at the top of the screen and then scroll down by
+                // firstItemAfterOriginalGapOffsetFromTop without intruding into unmeasured area
+                var cur = indexPathOfItemAfterOriginalGap
+                var amountScrolledUp: CGFloat = 0
+                while true {
+                    if cur.row <= 0 {
+                        break
+                    }
+                    if let cell = self.collectionView.cellForItem(at: indexPathOfItemAfterOriginalGap),
+                       cell.convert(.zero, to: self.view).y - self.view.safeAreaInsets.top > firstItemAfterOriginalGapOffsetFromTop {
+                        break
+                    }
+                    cur = IndexPath(row: cur.row - 1, section: cur.section)
+                    self.collectionView.scrollToItem(at: cur, at: .top, animated: false)
+                    self.collectionView.layoutIfNeeded()
+                    let attrs = self.collectionView.layoutAttributesForItem(at: cur)!
+                    amountScrolledUp += attrs.size.height
+                }
+                self.collectionView.contentOffset.y += amountScrolledUp
+                self.collectionView.contentOffset.y -= firstItemAfterOriginalGapOffsetFromTop
+            }
+            
+            snapshotView.removeFromSuperview()
+        }
+    }
+    
 }
 
 extension TimelineViewController {
@@ -539,29 +575,8 @@ extension TimelineViewController {
             }
             
             if addedItems {
-                // use a snapshot of the collection view to hide the flicker as the content offset changes and then changes back
+                let firstItemAfterOriginalGap = statusItems[gapIndex + 1]
-                let snapshotView = collectionView.snapshotView(afterScreenUpdates: false)!
+                applySnapshot(snapshot, maintainingBottomRelativeScrollPositionOf: firstItemAfterOriginalGap)
-                snapshotView.layer.zPosition = 1000
-                snapshotView.frame = view.bounds
-                view.addSubview(snapshotView)
-                
-                // Yes, these are all load bearing. Setting the contentOffset seems to cause the collection view to recalculate the size
-                // of some cells, thus changing the contentSize and the offset necessary to match to match the bottom offset.
-                // Three DispatchQueue.main.async's seems to be the fewest we can reliably get away with.
-                let bottomOffset = collectionView.contentSize.height - collectionView.contentOffset.y
-                dataSource.apply(snapshot, animatingDifferences: false) {
-                    self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
-                    DispatchQueue.main.async {
-                        self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
-                        DispatchQueue.main.async {
-                            self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
-                            DispatchQueue.main.async {
-                                self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
-                                snapshotView.removeFromSuperview()
-                            }
-                        }
-                    }
-                }
             } else {
                 dataSource.apply(snapshot, animatingDifferences: true) {}
             }