Add default WhenActivated calls to WPF base classes

src/ReactiveUI.Wpf/Common/ReactivePage.cs

@@ -160,6 +160,15 @@ protected ReactivePage(IntPtr handle)
     {
     }
 #endif
+#else
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ReactivePage{TViewModel}"/> class.
+    /// </summary>
+    public ReactivePage() => this.WhenActivated(disposables =>
+                                  {
+                                      // No-op, but ensures that when the Page is activated,
+                                      // any IActivatableViewModel logic in the ViewModel is also triggered.
+                                  });
 #endif
 
     /// <summary>

src/ReactiveUI.Wpf/Common/ReactiveUserControl.cs

@@ -144,6 +144,15 @@ protected ReactiveUserControl(IntPtr handle)
     {
     }
 #endif
+#else
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ReactiveUserControl{TViewModel}"/> class.
+    /// </summary>
+    public ReactiveUserControl() => this.WhenActivated(disposables =>
+                                         {
+                                             // No-op, but ensures that when the Page is activated,
+                                             // any IActivatableViewModel logic in the ViewModel is also triggered.
+                                         });
 #endif
 
     /// <summary>

src/ReactiveUI.Wpf/ReactiveWindow.cs

@@ -53,6 +53,18 @@ public class ReactiveWindow<TViewModel> :
                                     typeof(ReactiveWindow<TViewModel>),
                                     new PropertyMetadata(null));
 
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ReactiveWindow{TViewModel}"/> class.
+    /// </summary>
+    /// <remarks>When the window is activated, this constructor ensures that the ViewModel's activation logic
+    /// is also triggered if the ViewModel implements IActivatableViewModel. This enables coordinated activation and
+    /// deactivation of resources tied to the window and its ViewModel.</remarks>
+    public ReactiveWindow() => this.WhenActivated(disposables =>
+                                    {
+                                        // No-op, but ensures that when the Page is activated,
+                                        // any IActivatableViewModel logic in the ViewModel is also triggered.
+                                    });
+
     /// <summary>
     /// Gets the binding root view model.
     /// </summary>