I also encountered this issue. When I set: splitVC.preferredSplitBehavior = UISplitViewControllerSplitBehaviorTile; splitVC.preferredDisplayMode = UISplitViewControllerDisplayModeOneBesideSecondary; the primary view controller still overlays the secondary one. On macOS 15 / Xcode 16 this worked as expected (side-by-side), but on macOS 15 / Xcode 26 the behavior changed and the primary view now covers the secondary. This seems to be a regression or a new behavior introduced in the latest SDK.