From aec4501e2d539f5881d0fe48a54fc2df02fc148d Mon Sep 17 00:00:00 2001
From: Bryan Keller <bryan.keller@airbnb.com>
Date: Wed, 19 Nov 2025 13:00:24 -0800
Subject: [PATCH] Fix targetContentOffsetAnchor calculation

---
 MagazineLayout/LayoutCore/LayoutState.swift   | 10 ++---
 .../LayoutStateTargetContentOffsetTests.swift | 43 +++++++++++++++++++
 2 files changed, 47 insertions(+), 6 deletions(-)

diff --git a/MagazineLayout/LayoutCore/LayoutState.swift b/MagazineLayout/LayoutCore/LayoutState.swift
index 58c9d6c..2bcefb2 100644
--- a/MagazineLayout/LayoutCore/LayoutState.swift
+++ b/MagazineLayout/LayoutCore/LayoutState.swift
@@ -64,22 +64,20 @@ struct LayoutState {
   var targetContentOffsetAnchor: TargetContentOffsetAnchor {
     var visibleItemLocationFramePairs = [ElementLocationFramePair]()
     for itemLocationFramePair in modelState.itemLocationFramePairs(forItemsIn: bounds) {
-      // Only consider fully-visible items
-      guard bounds.contains(itemLocationFramePair.frame) else { continue }
       visibleItemLocationFramePairs.append(itemLocationFramePair)
     }
     visibleItemLocationFramePairs.sort { $0.elementLocation < $1.elementLocation }
 
     let firstVisibleItemLocationFramePair = visibleItemLocationFramePairs.first {
       // When scrolling up, only calculate a target content offset based on visible, already-sized
-      // cells. Otherwise, scrolling will be jumpy.
-      modelState.isItemHeightSettled(indexPath: $0.elementLocation.indexPath)
+      // cells. Otherwise, scrolling will be jumpy. Also, only consider fully-visible items.
+      modelState.isItemHeightSettled(indexPath: $0.elementLocation.indexPath) && bounds.contains($0.frame)
     } ?? visibleItemLocationFramePairs.first // fallback to the first item if we can't find one with a settled height
 
     let lastVisibleItemLocationFramePair = visibleItemLocationFramePairs.last {
       // When scrolling down, only calculate a target content offset based on visible, already-sized
-      // cells. Otherwise, scrolling will be jumpy.
-      modelState.isItemHeightSettled(indexPath: $0.elementLocation.indexPath)
+      // cells. Otherwise, scrolling will be jumpy. Also, only consider fully-visible items.
+      modelState.isItemHeightSettled(indexPath: $0.elementLocation.indexPath) && bounds.contains($0.frame)
     } ?? visibleItemLocationFramePairs.last // fallback to the last item if we can't find one with a settled height
 
     guard
diff --git a/Tests/LayoutStateTargetContentOffsetTests.swift b/Tests/LayoutStateTargetContentOffsetTests.swift
index 7367ce9..2441158 100644
--- a/Tests/LayoutStateTargetContentOffsetTests.swift
+++ b/Tests/LayoutStateTargetContentOffsetTests.swift
@@ -65,6 +65,23 @@ final class LayoutStateTargetContentOffsetTests: XCTestCase {
     XCTAssert(layoutState.targetContentOffsetAnchor == .topItem(id: id, distanceFromTop: 25))
   }
 
+  func testAnchor_TopToBottom_NoFullyVisibleCells_UsesFallback() throws {
+    // Create a model state with very large items (500px each) that are taller than the bounds (400px)
+    let bounds = CGRect(x: 0, y: 250, width: 300, height: 400)
+    let modelState = modelStateWithLargeItems(bounds: bounds)
+    let layoutState = LayoutState(
+      modelState: modelState,
+      bounds: bounds,
+      contentInset: UIEdgeInsets(top: 50, left: 0, bottom: 30, right: 0),
+      scale: 1,
+      verticalLayoutDirection: .topToBottom)
+
+    // Since no items are fully visible, the fallback should use the first partially visible item
+    // instead of returning .top or .bottom
+    let id = layoutState.modelState.idForItemModel(at: IndexPath(item: 0, section: 0))!
+    XCTAssert(layoutState.targetContentOffsetAnchor == .topItem(id: id, distanceFromTop: -300))
+  }
+
   // MARK: Bottom-to-Top Anchor Tests
 
   func testAnchor_BottomToTop_ScrolledToTop() throws {
@@ -243,6 +260,32 @@ final class LayoutStateTargetContentOffsetTests: XCTestCase {
     return modelState
   }
 
+  private func modelStateWithLargeItems(bounds: CGRect) -> ModelState {
+    let modelState = ModelState(currentVisibleBoundsProvider: { bounds })
+    let sections = [
+      SectionModel(
+        itemModels: [
+          // Create items that are 500px tall, larger than the 400px bounds height
+          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
+          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
+          ItemModel(widthMode: .fullWidth(respectsHorizontalInsets: true), preferredHeight: 500),
+        ],
+        headerModel: nil,
+        footerModel: nil,
+        backgroundModel: nil,
+        metrics: MagazineLayoutSectionMetrics(
+          collectionViewWidth: bounds.width,
+          collectionViewContentInset: .zero,
+          verticalSpacing: 0,
+          horizontalSpacing: 0,
+          sectionInsets: .zero,
+          itemInsets: .zero,
+          scale: 1))
+    ]
+    modelState.setSections(sections)
+    return modelState
+  }
+
 }
 
 // MARK: - ItemModel