Trivial Catalyst test app to see if `primaryBackgroundStyle = .sidebar` works to get a translucent blurry sidebar on Big Sur

SidebarTest.swift

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
}

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    private var _window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        let windowScene = scene as! UIWindowScene

        let window = UIWindow(windowScene: windowScene)
        _window = window

        let splitView = UISplitViewController()
        splitView.primaryBackgroundStyle = .sidebar
        splitView.viewControllers = [
            UIViewController(),
            UIViewController(),
        ]

        window.rootViewController = splitView
        window.makeKeyAndVisible()
    }
}