Add support for complex navigation in NavigationView

Sources/Core/View/NavigationView.swift

@@ -31,36 +31,38 @@ public struct NavigationView<Item>: WinUIWidget where Item: NavigationViewItem {
     /// The label for the settings item.
     var settingsLabel: String?
 
-    /// The currently selected item.
-    var selectedItem: Selection {
+    /// The currently selected flat navigation item.
+    var selectedItem: Selection? {
         get {
-            if let last = path.last(
-                where: { item in
-                    switch item {
-                    case .settings:
-                        return settings
-                    case let .custom(item):
-                        return items.contains(item)
-                    }
+            if path.isEmpty, let first = items.first {
+                path.append(.custom(item: first))
+            }
+            return path.last { component in
+                switch component {
+                case .settings:
+                    true
+                case let .custom(item):
+                    items.contains(item)
                 }
-            ) {
-                last
-            } else if let first = items.first {
-                .custom(item: first)
-            } else {
-                .settings
             }
         }
         nonmutating set {
-            path.append(newValue)
+            if let newValue {
+                path.append(newValue)
+            }
         }
     }
 
+    /// The currently selected complex navigation item.
+    var complexItem: Selection? {
+        path.last
+    }
+
     /// The view's header.
     var header: AnyView? {
         if let settingsLabel {
             Grid {
-                Text(selectedItem.description ?? settingsLabel)
+                Text(complexItem?.description ?? settingsLabel)
                 ModifierWrapper(
                     view: CommandBar {
                         primaryCommands
@@ -219,7 +221,7 @@ public struct NavigationView<Item>: WinUIWidget where Item: NavigationViewItem {
             navigationView.selectedItem = navigationView.menuItems
                 .first { ($0 as? WinUI.NavigationViewItem)?.name as? String == name.description } as Any?
         }
-        navigationView.isBackEnabled = !path.isEmpty
+        navigationView.isBackEnabled = path.count > 1
         storage.previousState = self
     }