I have a UISplitViewController whose secondary (detail) VC is a UICollectionViewController. I want the cells of the collection to resize based on changes to the size and aspect of the collection view. I trigger this resizing by overriding the UIContentContainer protocol method:
// MARK: - UIContentContainer protocol methods
func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator)
1) Not a bug; displayModeChange is not being treated a sizeTransition
2) Your UISplitviewController most likely already has a UISplitViewControllerDelegate which can implement the optional:
splitViewController(_ svc: UISplitViewController, willChangeToDisplayMode displayMode: UISplitViewControllerDisplayMode)
method that will get called with
UISplitViewControllerDisplayModeAllVisible depending on which mode the splitView is switching to.