Fix initial layout attributes when applying target content offset

MagazineLayout/Public/MagazineLayout.swift

@@ -266,6 +266,8 @@ public final class MagazineLayout: UICollectionViewLayout {
     }
     stagedContentOffsetAdjustment = nil
 
+    targetContentOffsetCompensatingYOffsetForAppearingItem = nil
+
     super.finalizeCollectionViewUpdates()
   }
 
@@ -395,7 +397,8 @@ public final class MagazineLayout: UICollectionViewLayout {
     }
 
     let itemFrame = modelState.frameForItem(at: itemLocation, .afterUpdates)
-    return itemLayoutAttributes(for: itemLocation, frame: itemFrame)
+    let attributes = itemLayoutAttributes(for: itemLocation, frame: itemFrame)
+    return attributes
   }
 
   override public func layoutAttributesForSupplementaryView(
@@ -449,6 +452,9 @@ public final class MagazineLayout: UICollectionViewLayout {
           initialLayoutAttributesForInsertedItemAt: itemIndexPath,
           byModifying: $0)
       }
+
+      attributes?.frame.origin.y += targetContentOffsetCompensatingYOffsetForAppearingItem ?? 0
+
       itemLayoutAttributesForPendingAnimations[itemIndexPath] = attributes
       return attributes
     } else if
@@ -459,7 +465,7 @@ public final class MagazineLayout: UICollectionViewLayout {
     {
       return previousLayoutAttributesForItem(at: initialIndexPath)
     } else {
-      return super.layoutAttributesForItem(at: itemIndexPath)
+      return super.initialLayoutAttributesForAppearingItem(at: itemIndexPath)
     }
   }
 
@@ -490,7 +496,7 @@ public final class MagazineLayout: UICollectionViewLayout {
       itemLayoutAttributesForPendingAnimations[finalIndexPath] = attributes
       return attributes
     } else {
-      return super.layoutAttributesForItem(at: itemIndexPath)
+      return super.finalLayoutAttributesForDisappearingItem(at: itemIndexPath)
     }
   }
 
@@ -691,10 +697,12 @@ public final class MagazineLayout: UICollectionViewLayout {
         contentOffsetAdjustment = nil
       }
 
-      let layoutAttributesForPendingAnimation = itemLayoutAttributesForPendingAnimations[preferredAttributes.indexPath]
-      layoutAttributesForPendingAnimation?.frame.size.height = modelState.frameForItem(
-        at: ElementLocation(indexPath: preferredAttributes.indexPath),
-        .afterUpdates).height
+      if let attributes = itemLayoutAttributesForPendingAnimations[preferredAttributes.indexPath] {
+        attributes.frame.size.height = modelState.frameForItem(
+          at: ElementLocation(indexPath: preferredAttributes.indexPath),
+          .afterUpdates).height
+        attributes.frame.origin.y -= (contentOffsetAdjustment?.y ?? 0)
+      }
 
     case .supplementaryView:
       contentOffsetAdjustment = nil
@@ -755,6 +763,7 @@ public final class MagazineLayout: UICollectionViewLayout {
   override public func invalidateLayout(with context: UICollectionViewLayoutInvalidationContext) {
     guard let context = context as? MagazineLayoutInvalidationContext else {
       assertionFailure("`context` must be an instance of `MagazineLayoutInvalidationContext`")
+      super.invalidateLayout(with: context)
       return
     }
 
@@ -797,6 +806,9 @@ public final class MagazineLayout: UICollectionViewLayout {
     }
 
     let yOffset = yOffset(for: targetContentOffsetAnchor)
+
+    targetContentOffsetCompensatingYOffsetForAppearingItem = proposedContentOffset.y - yOffset
+
     return CGPoint(x: proposedContentOffset.x, y: yOffset)
   }
 
@@ -840,6 +852,7 @@ public final class MagazineLayout: UICollectionViewLayout {
   // animations will start from the estimated height.
   private var itemLayoutAttributesForPendingAnimations = [IndexPath: UICollectionViewLayoutAttributes]()
   private var supplementaryViewLayoutAttributesForPendingAnimations = [IndexPath: UICollectionViewLayoutAttributes]()
+  private var targetContentOffsetCompensatingYOffsetForAppearingItem: CGFloat?
 
   private struct PrepareActions: OptionSet {
     let rawValue: UInt
@@ -862,13 +875,6 @@ public final class MagazineLayout: UICollectionViewLayout {
   // Used to provide the model state with the current visible bounds for the sole purpose of
   // supporting pinned headers and footers.
   private var currentVisibleBounds: CGRect {
-    let contentInset: UIEdgeInsets
-    if #available(iOS 11.0, tvOS 11.0, *) {
-      contentInset = currentCollectionView.adjustedContentInset
-    } else {
-      contentInset = currentCollectionView.contentInset
-    }
-
     let refreshControlHeight: CGFloat
     #if os(iOS)
     if