AppKit Modernization: NSTextSelectionManager & State Restoration

 import AppKit
 
−// Pre-macOS 27 approach: manual state saving via UserDefaults,
−// no NSWindowRestoration, window position not automatically preserved.
+// MARK: - Restoration Class
+class MainWindowRestoration: NSObject, NSWindowRestoration {
+    static func restoreWindow(
+        withIdentifier identifier: NSUserInterfaceItemIdentifier,
+        state: NSCoder,
+        completionHandler: @escaping (NSWindow?, Error?) -> Void
+    ) {
+        let appDelegate = NSApp.delegate as! AppDelegate
+        if identifier.rawValue == "MainWindow" {
+            // Reuse the existing main window
+            completionHandler(appDelegate.mainWindowController.window, nil)
+        } else {
+            let error = NSError(
+                domain: "com.example.app",
+                code: 1,
+                userInfo: [NSLocalizedDescriptionKey: "Unknown window identifier"]
+            )
+            completionHandler(nil, error)
+        }
+    }
+}
 
−class LegacyWindowController: NSWindowController {
+// MARK: - Window Controller
+class MainWindowController: NSWindowController {
     private let selectedProductKey = "selectedProductIdentifier"
+    var selectedProductIdentifier: String = "product-1"
 
     override func windowDidLoad() {
         super.windowDidLoad()
−        // Restore from UserDefaults manually
−        if let saved = UserDefaults.standard.string(forKey: selectedProductKey) {
−            (contentViewController as? LegacyProductViewController)?.select(productID: saved)
−        }
+        guard let window else { return }
+        // Enable state restoration
+        window.identifier = NSUserInterfaceItemIdentifier("MainWindow")
+        window.isRestorable = true
+        window.restorationClass = MainWindowRestoration.self
+        window.setFrameAutosaveName("MainWindow")
     }
 
     func selectProduct(_ identifier: String) {
−        // Save immediately to UserDefaults — no invalidation model
−        UserDefaults.standard.set(identifier, forKey: selectedProductKey)
+        selectedProductIdentifier = identifier
+        // Notify AppKit that restorable state has changed
+        invalidateRestorableState()
     }
+
+    override func encodeRestorableState(with coder: NSCoder) {
+        super.encodeRestorableState(with: coder)
+        coder.encode(selectedProductIdentifier, forKey: selectedProductKey)
+    }
+
+    override func restoreState(with coder: NSCoder) {
+        super.restoreState(with: coder)
+        if let identifier = coder.decodeObject(forKey: selectedProductKey) as? String {
+            selectedProductIdentifier = identifier
+            // Push restored selection into your view controller here
+            (contentViewController as? ProductViewController)?.select(productID: identifier)
+        }
+    }
 }
 
−class LegacyProductViewController: NSViewController {
+// MARK: - Stub view controller
+class ProductViewController: NSViewController {
     func select(productID: String) {
−        print("Restored selection (legacy): \(productID)")
+        // Update UI to reflect restored selection
+        print("Restored selection: \(productID)")
     }
+}
+
+// MARK: - AppDelegate stub
+class AppDelegate: NSObject, NSApplicationDelegate {
+    let mainWindowController = MainWindowController(windowNibName: "MainWindow")
 }