Adjust content offset when adjusted content insets change

MagazineLayout.xcodeproj/project.pbxproj

@@ -7,6 +7,7 @@
 	objects = {
 
 /* Begin PBXBuildFile section */
+		60432D952E05DB41001728F0 /* ContentInsetAdjustingContentOffsetTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 60432D942E05DB41001728F0 /* ContentInsetAdjustingContentOffsetTests.swift */; };
 		9332FB0822969B5600483D99 /* RowOffsetTrackerTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9332FB0622969AB200483D99 /* RowOffsetTrackerTests.swift */; };
 		93424B012256878B003D00C0 /* MagazineLayoutFooterVisibilityMode.swift in Sources */ = {isa = PBXBuildFile; fileRef = 93424B002256878B003D00C0 /* MagazineLayoutFooterVisibilityMode.swift */; };
 		93540AB0282E25D90008BD6F /* ScreenPixelAlignment.swift in Sources */ = {isa = PBXBuildFile; fileRef = 93540AAF282E25D90008BD6F /* ScreenPixelAlignment.swift */; };
@@ -58,6 +59,7 @@
 /* End PBXContainerItemProxy section */
 
 /* Begin PBXFileReference section */
+		60432D942E05DB41001728F0 /* ContentInsetAdjustingContentOffsetTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ContentInsetAdjustingContentOffsetTests.swift; sourceTree = "<group>"; };
 		9332FB0622969AB200483D99 /* RowOffsetTrackerTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = RowOffsetTrackerTests.swift; sourceTree = "<group>"; };
 		93424B002256878B003D00C0 /* MagazineLayoutFooterVisibilityMode.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = MagazineLayoutFooterVisibilityMode.swift; sourceTree = "<group>"; };
 		93540AAF282E25D90008BD6F /* ScreenPixelAlignment.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ScreenPixelAlignment.swift; sourceTree = "<group>"; };
@@ -156,6 +158,7 @@
 				9332FB0622969AB200483D99 /* RowOffsetTrackerTests.swift */,
 				93A1C00E21ACED0100DED67D /* TestingSupport.swift */,
 				93540AB1282E26340008BD6F /* ScreenPixelAlignmentTests.swift */,
+				60432D942E05DB41001728F0 /* ContentInsetAdjustingContentOffsetTests.swift */,
 			);
 			path = Tests;
 			sourceTree = "<group>";
@@ -382,6 +385,7 @@
 			files = (
 				FDE08E162B2CC47800C9D24D /* TargetContentOffsetAnchorTests.swift in Sources */,
 				93A1C04B21ACED1100DED67D /* ModelStateInitiallSetUpTests.swift in Sources */,
+				60432D952E05DB41001728F0 /* ContentInsetAdjustingContentOffsetTests.swift in Sources */,
 				9332FB0822969B5600483D99 /* RowOffsetTrackerTests.swift in Sources */,
 				93A1C04C21ACED1100DED67D /* ModelStateLayoutTests.swift in Sources */,
 				93A1C04921ACED1100DED67D /* ModelStateEmptySectionLayoutTests.swift in Sources */,

MagazineLayout/Public/MagazineLayout.swift

@@ -787,6 +787,22 @@ public final class MagazineLayout: UICollectionViewLayout {
       return
     }
 
+    // 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.
+    if
+      case .bottomToTop = verticalLayoutDirection,
+      let previousContentInset
+    {
+      if previousContentInset.top != contentInset.top {
+        context.contentOffsetAdjustment.y += contentInset.top - previousContentInset.top
+      }
+      if previousContentInset.bottom != contentInset.bottom {
+        context.contentOffsetAdjustment.y += contentInset.bottom - previousContentInset.bottom
+      }
+    }
+    previousContentInset = contentInset
+
     let shouldInvalidateLayoutMetrics = !context.invalidateEverything &&
       !context.invalidateDataSourceCounts
 
@@ -899,6 +915,7 @@ public final class MagazineLayout: UICollectionViewLayout {
   private var isPerformingAnimatedBoundsChange = false
   private var targetContentOffsetAnchor: TargetContentOffsetAnchor?
   private var stagedContentOffsetAdjustment: CGPoint?
+  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.

Tests/ContentInsetAdjustingContentOffsetTests.swift

@@ -0,0 +1,84 @@
+// Created by Bryn Bodayle on 6/20/25.
+// Copyright © 2025 Airbnb Inc. All rights reserved.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import XCTest
+@testable import MagazineLayout
+
+final class ContentInsetAdjustingContentOffsetTests: XCTestCase {
+
+  func testContentOffsetIsNotAdjustedForTopInsetChangeWithToTopBottomLayout() {
+    let layout = MagazineLayout(verticalLayoutDirection: .topToBottom)
+    let collectionView = StubCollectionView(
+      frame: .zero,
+      collectionViewLayout: layout)
+    let context = MagazineLayoutInvalidationContext()
+    layout.invalidateLayout(with: context)
+    XCTAssertEqual(context.contentOffsetAdjustment, .zero)
+
+    collectionView.stubAdjustedContentInset = .init(top: 50, left: 0, bottom: 50, right: 0)
+    layout.invalidateLayout(with: context)
+    XCTAssertEqual(context.contentOffsetAdjustment, .zero)
+  }
+
+  func testContentOffsetIsAdjustedForTopInsetChangeWithBottomToTopLayout() {
+    let layout = MagazineLayout(verticalLayoutDirection: .bottomToTop)
+    let collectionView = StubCollectionView(
+      frame: .zero,
+      collectionViewLayout: layout)
+    let context = MagazineLayoutInvalidationContext()
+    layout.invalidateLayout(with: context)
+    XCTAssertEqual(context.contentOffsetAdjustment, .zero)
+
+    collectionView.stubAdjustedContentInset = .init(top: 50, left: 0, bottom: 0, right: 0)
+    layout.invalidateLayout(with: context)
+    XCTAssertEqual(context.contentOffsetAdjustment, .init(x: 0, y: 50))
+  }
+
+  func testContentOffsetIsAdjustedForBottomInsetChangeWithBottomToTopLayout() {
+    let layout = MagazineLayout(verticalLayoutDirection: .bottomToTop)
+    let collectionView = StubCollectionView(
+      frame: .zero,
+      collectionViewLayout: layout)
+    let context = MagazineLayoutInvalidationContext()
+    layout.invalidateLayout(with: context)
+    XCTAssertEqual(context.contentOffsetAdjustment, .zero)
+
+    collectionView.stubAdjustedContentInset = .init(top: 0, left: 0, bottom: 75, right: 0)
+    layout.invalidateLayout(with: context)
+    XCTAssertEqual(context.contentOffsetAdjustment, .init(x: 0, y: 75))
+  }
+
+  func testContentOffsetIsAdjustedForTopAndBottomInsetChangesWithBottomToTopLayout() {
+    let layout = MagazineLayout(verticalLayoutDirection: .bottomToTop)
+    let collectionView = StubCollectionView(
+      frame: .zero,
+      collectionViewLayout: layout)
+    let context = MagazineLayoutInvalidationContext()
+    layout.invalidateLayout(with: context)
+    XCTAssertEqual(context.contentOffsetAdjustment, .zero)
+
+    collectionView.stubAdjustedContentInset = .init(top: 100, left: 0, bottom: 100, right: 0)
+    layout.invalidateLayout(with: context)
+    XCTAssertEqual(context.contentOffsetAdjustment, .init(x: 0, y: 200))
+  }
+}
+
+private class StubCollectionView: UICollectionView {
+
+  var stubAdjustedContentInset: UIEdgeInsets = .zero
+  override var adjustedContentInset: UIEdgeInsets {
+    stubAdjustedContentInset
+  }
+}

Tests/ScreenPixelAlignmentTests.swift

@@ -1,9 +1,6 @@
 // Created by Bryan Keller on 5/12/22.
 // Copyright © 2022 Airbnb Inc. All rights reserved.
 
-// Created by Bryan Keller on 3/31/20.
-// Copyright © 2020 Airbnb Inc. All rights reserved.
-
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at