Fix profile page switching on iOS 18

Tusker/Screens/Profile/ProfileHeaderCollectionViewCell.swift

@@ -38,10 +38,12 @@ class ProfileHeaderCollectionViewCell: UICollectionViewCell {
             header.translatesAutoresizingMaskIntoConstraints = false
             contentView.embedSubview(header)
             self.state = .view(header)
-        case .view(_):
+        case .view(let existing):
+            if existing !== header {
                 fatalError("profile header collection view cell already has view")
             }
         }
+    }
     
     func addConstraint(height: CGFloat) -> ProfileHeaderView? {
         switch state {

Tusker/Screens/Profile/ProfileStatusesViewController.swift

@@ -17,7 +17,7 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
     let filterer: Filterer
     private(set) var accountID: String!
     let kind: Kind
-    var initialHeaderMode: HeaderMode?
+    var headerViewMode: HeaderMode?
     weak var profileHeaderDelegate: ProfileHeaderViewDelegate?
     
     private(set) var controller: TimelineLikeController<TimelineItem>!
@@ -30,7 +30,9 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
         view as? UICollectionView
     }
     private(set) var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
-    private(set) var headerCell: ProfileHeaderCollectionViewCell?
+    var headerCell: ProfileHeaderCollectionViewCell? {
+        collectionView.cellForItem(at: IndexPath(item: 0, section: 0)) as? ProfileHeaderCollectionViewCell
+    }
     
     var reconfigureVisibleItemsOnEndDecelerating: Bool = false
     
@@ -173,29 +175,29 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
         return UICollectionViewDiffableDataSource(collectionView: collectionView) { [unowned self] collectionView, indexPath, itemIdentifier in
             switch itemIdentifier {
             case .header(let id):
-                if let headerCell = self.headerCell {
-                    headerCell.view?.updateUI(for: id)
-                    return headerCell
-                } else {
                 let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "headerCell", for: indexPath) as! ProfileHeaderCollectionViewCell
-                    switch self.initialHeaderMode {
+                switch self.headerViewMode {
                 case nil:
-                        fatalError("missing initialHeaderMode")
+                    fatalError("missing headerViewMode")
-                    case .createView:
+                case .createViewIfNeeded:
+                    if let view = cell.view {
+                        view.updateUI(for: id)
+                        self.headerViewMode = .useExistingView(view)
+                    } else {
                         let view = ProfileHeaderView.create()
                         view.delegate = self.profileHeaderDelegate
                         view.updateUI(for: id)
                         view.pagesSegmentedControl.setSelectedOption(self.owner!.currentPage, animated: false)
                         cell.addHeader(view)
+                        self.headerViewMode = .useExistingView(view)
+                    }
                 case .useExistingView(let view):
                     view.updateUI(for: id)
                     cell.addHeader(view)
                 case .placeholder(height: let height):
                     _ = cell.addConstraint(height: height)
                 }
-                    self.headerCell = cell
                 return cell
-                }
             case .status(id: let id, collapseState: let collapseState, filterState: let filterState, pinned: let pinned):
                 let (result, precomputedContent) = filterResult(state: filterState, statusID: id)
                 switch result {
@@ -411,7 +413,9 @@ extension ProfileStatusesViewController {
         case statuses, withReplies, onlyMedia
     }
     enum HeaderMode {
-        case createView, useExistingView(ProfileHeaderView), placeholder(height: CGFloat)
+        case createViewIfNeeded
+        case useExistingView(ProfileHeaderView)
+        case placeholder(height: CGFloat)
     }
 }
 

Tusker/Screens/Profile/ProfileViewController.swift

@@ -178,7 +178,7 @@ class ProfileViewController: UIViewController, StateRestorableViewController {
         guard let currentIndex else {
             assert(!animated)
             // if old doesn't exist, we're selecting the initial view controller, so moving the header around isn't necessary
-            new.initialHeaderMode = .createView
+            new.headerViewMode = .createViewIfNeeded
             new.view.translatesAutoresizingMaskIntoConstraints = false
             addChild(new)
             view.addSubview(new.view)
@@ -213,11 +213,14 @@ class ProfileViewController: UIViewController, StateRestorableViewController {
         
         // old header cell must have the header view
         let headerView = oldHeaderCell.addConstraint(height: oldHeaderCell.bounds.height)!
+        // Set the outgoing VC's header view mode to placeholder, so that it does steal the header view back
+        // in case it updates the cell in the background.
+        old.headerViewMode = .placeholder(height: oldHeaderCell.bounds.height)
         
         if let newHeaderCell = new.headerCell {
             _ = newHeaderCell.addConstraint(height: oldHeaderCell.bounds.height)
         } else {
-            new.initialHeaderMode = .placeholder(height: oldHeaderCell.bounds.height)
+            new.headerViewMode = .placeholder(height: oldHeaderCell.bounds.height)
         }
         
         // disable user interaction during animation, to avoid any potential weird race conditions
@@ -285,7 +288,7 @@ class ProfileViewController: UIViewController, StateRestorableViewController {
                 if let newHeaderCell = new.headerCell {
                     newHeaderCell.addHeader(headerView)
                 } else {
-                    new.initialHeaderMode = .useExistingView(headerView)
+                    new.headerViewMode = .useExistingView(headerView)
                 }
                 
                 self.state = .idle