Add offset adjustment behavior to top and bottom

Author: brynbodayle

MagazineLayout/Public/MagazineLayout.swift

@@ -787,18 +787,21 @@ public final class MagazineLayout: UICollectionViewLayout {
       return
     }
 
-    // If our layout direction is `bottomToTop`, allow changes to the bottom content inset to
-    // automatically adjust the content offset. `UICollectionView` behaves this way by default when
-    // the top content inset changes, so this adds the same behavior to the bottom.
-    let bottomContentInset = contentInset.bottom
+    // If our layout direction is `bottomToTop`, allow changes to the top and bottom content insets
+    // to automatically adjust the content offset. `UICollectionView` behaves this way by default
+    // when the top content inset changes, so this adds the same behavior to the bottom.
     if
       case .bottomToTop = verticalLayoutDirection,
-      let previousBottomContentInset,
-      previousBottomContentInset != bottomContentInset
+      let previousContentInset
     {
-      context.contentOffsetAdjustment.y = bottomContentInset - previousBottomContentInset
+      if previousContentInset.top != contentInset.top {
+        context.contentOffsetAdjustment.y += contentInset.top - previousContentInset.top
+      }
+      if previousContentInset.bottom != contentInset.bottom {
+        context.contentOffsetAdjustment.y += contentInset.bottom - previousContentInset.bottom
+      }
     }
-    previousBottomContentInset = bottomContentInset
+    previousContentInset = contentInset
 
     let shouldInvalidateLayoutMetrics = !context.invalidateEverything &&
       !context.invalidateDataSourceCounts
@@ -912,7 +915,7 @@ public final class MagazineLayout: UICollectionViewLayout {
   private var isPerformingAnimatedBoundsChange = false
   private var targetContentOffsetAnchor: TargetContentOffsetAnchor?
   private var stagedContentOffsetAdjustment: CGPoint?
-  private var previousBottomContentInset: CGFloat?
+  private var previousContentInset: UIEdgeInsets?
 
   // Used to provide the model state with the current visible bounds for the sole purpose of
   // supporting pinned headers and footers.