How to turn off background extension effect in UISplitViewController?

I have a triple-column UISplitViewController setup in "tile" mode. Each of the 3 columns has a table view controller. Under iPadOS 26, the section headers and row selection in the middle table extends all the way to the left of the screen, behind the primary column. It looks terrible. The documentation for "Adopting Liquid Glass" makes it sound like you can add this behavior by using UIBackgroundExtensionView. But I get this behavior automatically in a UISplitViewController. How do I turn this off?

I created a simpler sample using a double-column split view with two table view controllers. Here's a screenshot of the result:

Note how the section headers and the row selection appear all the way to the left edge of the screen. I don't want that effect. How do you turn off this effect in a UISplitViewController?

Here is the code used to setup the split view and the app's main window:

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    guard let winScene = (scene as? UIWindowScene) else { return }

    let primary = PrimaryViewController(style: .plain)
    let primaryNC = UINavigationController(rootViewController: primary)
    let detail = DetailViewController(style: .plain)
    let detailNC = UINavigationController(rootViewController: detail)

    let sv = UISplitViewController(style: .doubleColumn)
    sv.preferredDisplayMode = .oneBesideSecondary
    sv.preferredSplitBehavior = .tile
    sv.primaryBackgroundStyle = .none
    sv.displayModeButtonVisibility = .automatic

    sv.setViewController(primaryNC, for: .primary)
    sv.setViewController(detailNC, for: .secondary)

    let win = UIWindow(windowScene: winScene)
    win.rootViewController = sv
    win.makeKeyAndVisible()
    window = win
}

The PrimaryViewController and DetailViewController are simple UITableViewController subclasses that only add a few rows and section headers as needed.