More fiddling with how Jump to Present works

Now, when loading present items, they're inserted into the data source
immediately along with a gap. If the user taps Jump to Present, then a
new snapshot _with only the present items_ will be applied (which allows
infinite scrolling to work properly when they scroll back down) and the
view scrolled-to-top. Tapping Go Back, then, applies the original
snapshot (i.e., the current one from when Jump to Present was tapped)
and restores the scroll position.

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -412,7 +412,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                 // I'm not sure whether this should move into TimelineLikeController/TimelineLikeCollectionViewController
                 let (_, presentItems) = await (loadNewerAndEndRefreshing(), try? loadInitial())
                 if let presentItems, !presentItems.isEmpty {
-                    insertPresentItemsIfNecessary(presentItems)
+                    insertPresentItemsAndShowJumpToast(presentItems)
                 }
             }
         }
@@ -430,13 +430,13 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                     UIAccessibility.post(notification: .screenChanged, argument: self.collectionView.cellForItem(at: IndexPath(row: 0, section: 0)))
                 }
             } else {
-                insertPresentItemsIfNecessary(presentItems)
+                insertPresentItemsAndShowJumpToast(presentItems)
             }
         }
     }
     
-    private func insertPresentItemsIfNecessary(_ presentItems: [String]) {
+    private func insertPresentItemsAndShowJumpToast(_ presentItems: [String]) {
-        let snapshot = dataSource.snapshot()
+        var snapshot = dataSource.snapshot()
         guard snapshot.indexOfSection(.statuses) != nil else {
             return
         }
@@ -444,13 +444,38 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         if case .status(id: let firstID, _, _) = currentItems.first,
            // if there's no overlap between presentItems and the existing items in the data source, prompt the user
            !presentItems.contains(firstID) {
+            let applySnapshotBeforeScrolling: Bool
             
-            // create a new snapshot to reset the timeline to the "present" state
+            // remove any existing gap, if there is one
-            var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
+            if let index = currentItems.lastIndex(of: .gap) {
-            snapshot.appendSections([.statuses])
+                snapshot.deleteItems(Array(currentItems[index...]))
-            snapshot.appendItems(presentItems.map { .status(id: $0, collapseState: .unknown, filterState: .unknown) }, toSection: .statuses)
                 
-            var config = ToastConfiguration(title: "Jump to present")
+                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
+                }
+            } else {
+                // there is no existing gap
+                applySnapshotBeforeScrolling = true
+            }
+            snapshot.insertItems([.gap], beforeItem: currentItems.first!)
+            snapshot.insertItems(presentItems.map { .status(id: $0, collapseState: .unknown, filterState: .unknown) }, beforeItem: .gap)
+            
+            if applySnapshotBeforeScrolling {
+                let firstVisibleIndexPath = collectionView.indexPathsForVisibleItems.min()!
+                let firstVisibleItem = dataSource.itemIdentifier(for: firstVisibleIndexPath)!
+                applySnapshot(snapshot, maintainingBottomRelativeScrollPositionOf: firstVisibleItem)
+            }
+            
+            var config = ToastConfiguration(title: "Jump to Present")
             config.edge = .top
             config.systemImageName = "arrow.up"
             config.dismissAutomaticallyAfter = 4
@@ -468,18 +493,23 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                     origItemAtTop = nil
                 }
 
-                self.dataSource.apply(snapshot, animatingDifferences: true) {
+                // when the user explicitly taps Jump to Present, we drop all the old items to let infinite scrolling work properly when they scroll back down
+                var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
+                snapshot.appendSections([.statuses])
+                snapshot.appendItems(presentItems.map { .status(id: $0, collapseState: .unknown, filterState: .unknown) }, toSection: .statuses)
+                // don't animate the snapshot change, the scrolling animation will paper over the switch
+                self.dataSource.apply(snapshot, animatingDifferences: false) {
                     self.collectionView.scrollToItem(at: IndexPath(row: 0, section: 0), at: .top, animated: true)
                     
-                    var config = ToastConfiguration(title: "Go back")
+                    var config = ToastConfiguration(title: "Go Back")
                     config.edge = .top
                     config.systemImageName = "arrow.down"
-                    config.dismissAutomaticallyAfter = 4
+                    config.dismissAutomaticallyAfter = 2
                     config.action = { [unowned self] toast in
                         toast.dismissToast(animated: true)
                         // todo: it would be nice if we could animate this, but that doesn't work with the screen-position-maintaining stuff
                         if let (item, offset) = origItemAtTop {
-                            self.applySnapshot(snapshot, maintainingScreenPosition: offset, ofItem: item)
+                            self.applySnapshot(origSnapshot, maintainingScreenPosition: offset, ofItem: item)
                         } else {
                             self.dataSource.apply(origSnapshot, animatingDifferences: false)
                         }
@@ -516,12 +546,17 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                 // firstItemAfterOriginalGapOffsetFromTop without intruding into unmeasured area
                 var cur = indexPathOfItemToMaintain
                 var amountScrolledUp: CGFloat = 0
+                var first = true
                 while true {
                     if cur.row <= 0 {
                         break
                     }
                     if let cell = self.collectionView.cellForItem(at: indexPathOfItemToMaintain),
                        cell.convert(.zero, to: self.view).y - self.view.safeAreaInsets.top > offsetFromTop {
+                        // if we're breaking from the loop at the first iteration, we need to make sure to still call scrollToItem for the current row
+                        if first {
+                            self.collectionView.scrollToItem(at: cur, at: .top, animated: false)
+                        }
                         break
                     }
                     cur = IndexPath(row: cur.row - 1, section: cur.section)
@@ -529,6 +564,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                     self.collectionView.layoutIfNeeded()
                     let attrs = self.collectionView.layoutAttributesForItem(at: cur)!
                     amountScrolledUp += attrs.size.height
+                    first = false
                 }
                 self.collectionView.contentOffset.y += amountScrolledUp
                 self.collectionView.contentOffset.y -= offsetFromTop