Fix jumpy scrolling due to incorrect target content offset calculation (#137)

Author: bryankeller

MagazineLayout/LayoutCore/ModelState.swift

@@ -105,6 +105,17 @@ final class ModelState {
     return nil
   }
 
+  func isItemHeightSettled(indexPath: IndexPath) -> Bool {
+    let item = sectionModels(for: .afterUpdates)[indexPath.section].itemModel(
+      atIndex: indexPath.item)
+    switch item.sizeMode.heightMode {
+    case .static:
+      return true
+    case .dynamicAndStretchToTallestItemInRow, .dynamic(_):
+      return item.preferredHeight != nil
+    }
+  }
+
   func itemModelHeightModeDuringPreferredAttributesCheck(
     at indexPath: IndexPath)
     -> MagazineLayoutItemHeightMode?

MagazineLayout/Public/MagazineLayout.swift

@@ -1294,7 +1294,11 @@ public final class MagazineLayout: UICollectionViewLayout {
     }
     visibleItemLocationFramePairs.sort { $0.elementLocation < $1.elementLocation }
 
-    let firstVisibleItemLocationFramePair = visibleItemLocationFramePairs.first
+    let firstVisibleItemLocationFramePair = visibleItemLocationFramePairs.first {
+      // When scrolling up, only calculate a a target content offset based on visible, already-sized
+      // cells. Otherwise, scrolling will be jumpy.
+      modelState.isItemHeightSettled(indexPath: $0.elementLocation.indexPath)
+    }
     let lastVisibleItemLocationFramePair = visibleItemLocationFramePairs.last
 
     guard