A StackOverflow user suggested switching Xcode versions, and indeed, using 15.2 instead of 15.3 appears to have solved the problem.

I am sorry, I meant hang point, not "pont" . As far as more complete code goes, I don' t see much point, since it happens at initailization of a CGFloat instance variable, but here you go, code for the entire class: class FileOrFolderCell: UICollectionViewCell, CustomCell { @IBOutlet weak var emojiLabel: UILabel! @IBOutlet weak var nameLabel: MarqueeLabel! @IBOutlet weak var emojiBadge: UILabel! @IBOutlet weak var imageView: UIImageView! let emojiLabelInitialFontSize: CGFloat = 64 @IBOutlet weak var newBadge: UILabel! var url: URL! var thumbURL: URL? var isNew = false { didSet { newBadge.isHidden = !isNew } } var emoji = "" { didSet { emojiLabel.text = emoji emojiBadge.text = emoji } } func setFontSizesFor(width: CGFloat) { emojiLabel.font = emojiLabel.font?.withSize(32) newBadge.font = newBadge.font?.withSize(20) } } As you see, there is basically nothing there and whatever little this is, it all has to run on the UI thread (it is a subclass of UICollectionView).

My experience with this indicates that you also have to enclose the call to URL(resolvingBookmarkData:) between startAccessingSecurityScopedResource() and stopAccessingSecurityScopedResource() for it to work reliably. After much searching, I only found a passing reference to this fact somewhere on the web (which, to my knowledge, appears not to be properly documented) and suddenly, a lot of issues I was having went away as if by magic.