Fix System.InvalidCastException in PageNavigationHost (#21144)

* Fix invalid cast exception in PageNavigationHost

* Added tests

---------

Co-authored-by: Javier Suárez Ruiz <javiersuarezruiz@hotmail.com>

src/Avalonia.Controls/Page/PageNavigationHost.cs

@@ -114,8 +114,8 @@ namespace Avalonia.Controls
 
             if (change.Property == PageProperty)
             {
-                var oldPage = change.GetOldValue<Page?>();
-                var newPage = change.GetNewValue<Page?>();
+                var oldPage = change.OldValue as Page;
+                var newPage = change.NewValue as Page;
 
                 SetCurrentValue(ContentProperty, newPage);
 
@@ -132,10 +132,10 @@ namespace Avalonia.Controls
             if (e.Property != ContentPresenter.ChildProperty)
                 return;
 
-            if (e.GetOldValue<object?>() is Page oldPage)
+            if (e.OldValue is Page oldPage)
                 oldPage.SafeAreaPadding = default;
 
-            if (e.GetNewValue<object?>() is Page newPage && _insetManager != null)
+            if (e.NewValue is Page newPage && _insetManager != null)
                 newPage.SafeAreaPadding = _insetManager.SafeAreaPadding;
         }
 

tests/Avalonia.Controls.UnitTests/PageNavigationHostTests.cs

@@ -129,5 +129,38 @@ public class PageNavigationHostTests
             Assert.Equal("NavigatedFrom", order[0]);
             Assert.Equal("NavigatedTo",   order[1]);
         }
+
+        [Fact]
+        public void InitialLayout_WithExistingPage_DoesNotThrow_WhenContentPresenterChildIsAssigned()
+        {
+            var page = new ContentPage { Header = "Home" };
+            var host = new PageNavigationHost { Page = page };
+            var root = new TestRoot { Child = host };
+
+            var exception = Record.Exception(() => root.LayoutManager.ExecuteInitialLayoutPass());
+
+            Assert.Null(exception);
+            Assert.NotNull(host.Presenter);
+            Assert.Same(page, host.Presenter!.Child);
+        }
+
+        [Fact]
+        public void ReplacingPage_ResetsOldPresenterChildSafeAreaPadding()
+        {
+            var first = new ContentPage { Header = "First" };
+            var second = new ContentPage { Header = "Second" };
+            var host = new PageNavigationHost { Page = first };
+            var root = new TestRoot { Child = host };
+
+            root.LayoutManager.ExecuteInitialLayoutPass();
+            first.SafeAreaPadding = new Thickness(1, 2, 3, 4);
+
+            var exception = Record.Exception(() => host.Page = second);
+
+            Assert.Null(exception);
+            Assert.Equal(default, first.SafeAreaPadding);
+            Assert.NotNull(host.Presenter);
+            Assert.Same(second, host.Presenter!.Child);
+        }
     }
 }