fix: ensure indicator is visibility upon loaded.

demo/Ursa.Demo/ViewModels/SelectionListDemoViewModel.cs

@@ -14,6 +14,7 @@ public partial class SelectionListDemoViewModel: ObservableObject
         {
             "Ding", "Otter", "Husky", "Mr. 17", "Cass"
         };
+        SelectedItem = Items[0];
     }
 
     public void Clear()

src/Ursa/Controls/SelectionList/SelectionList.cs

@@ -7,6 +7,7 @@ using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Selection;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
+using Avalonia.Interactivity;
 using Avalonia.Rendering.Composition;
 using Avalonia.Rendering.Composition.Animations;
 using Irihi.Avalonia.Shared.Helpers;
@@ -95,6 +96,18 @@ public class SelectionList: SelectingItemsControl
         EnsureIndicatorAnimation();
     }
 
+    protected override void OnLoaded(RoutedEventArgs e)
+    {
+        base.OnLoaded(e);
+        if(_indicator is not null && SelectedItem is not null)
+        {
+            var container = ContainerFromItem(SelectedItem);
+            if (container is null) return;
+            _indicator.Opacity = 1;
+            _indicator.Arrange(container.Bounds);
+        }
+    }
+
     private void EnsureIndicatorAnimation()
     {
         if (_indicator is not null)