fix for failing tests on CarouselPage and TabbedPage

src/Avalonia.Controls/Page/CarouselPage.cs

@@ -116,6 +116,8 @@ namespace Avalonia.Controls
         {
             base.OnApplyTemplate(e);
 
+            var requestedIndex = SelectedIndex;
+
             if (_carousel != null)
             {
                 _carousel.SelectionChanged -= OnCarouselSelectionChanged;
@@ -126,7 +128,6 @@ namespace Avalonia.Controls
 
             if (_carousel != null)
             {
-                _carousel.SelectionChanged += OnCarouselSelectionChanged;
                 _carousel.ContainerPrepared += OnCarouselContainerPrepared;
                 _carousel.PageTransition = PageTransition;
                 _carousel.ItemsPanel = ItemsPanel;
@@ -134,11 +135,13 @@ namespace Avalonia.Controls
                 _carousel.IsSwipeEnabled = IsGestureEnabled;
                 _carousel.ItemsSource = (IEnumerable?)ItemsSource ?? Pages;
 
-                if (SelectedIndex >= 0)
+                if (requestedIndex >= 0)
                 {
-                    _carousel.SelectedIndex = SelectedIndex;
+                    _carousel.SelectedIndex = requestedIndex;
                 }
 
+                _carousel.SelectionChanged += OnCarouselSelectionChanged;
+
                 UpdateActivePage();
             }
         }

src/Avalonia.Controls/Page/TabbedPage.cs

@@ -179,6 +179,8 @@ namespace Avalonia.Controls
         {
             base.OnApplyTemplate(e);
 
+            var requestedIndex = SelectedIndex;
+
             if (_tabControl != null)
             {
                 _tabControl.SelectionChanged -= TabControl_SelectionChanged;
@@ -192,13 +194,14 @@ namespace Avalonia.Controls
 
             if (_tabControl != null)
             {
-                _tabControl.SelectionChanged += TabControl_SelectionChanged;
                 _tabControl.ContainerPrepared += OnContainerPrepared;
                 _tabControl.ContainerClearing += OnContainerClearing;
                 _tabControl.ItemsSource = (IEnumerable?)ItemsSource ?? Pages;
 
-                if (SelectedIndex >= 0)
-                    _tabControl.SelectedIndex = SelectedIndex;
+                if (requestedIndex >= 0)
+                    _tabControl.SelectedIndex = requestedIndex;
+
+                _tabControl.SelectionChanged += TabControl_SelectionChanged;
 
                 if (PageTransition != null)
                     _tabControl.PageTransition = PageTransition;