Fix SplitView :closed pseudoclass not being added on control initialization #17176 (#17178)

* test(SplitViewTests.cs): added failing test for issue #17176

* fix(SplitView.cs): added pane state pseudo class update in OnApplyTemplate

- fixed #17176

* test(SplitViewTests): added globalclock mock to failing test

src/Avalonia.Controls/SplitView/SplitView.cs

@@ -297,6 +297,7 @@ namespace Avalonia.Controls
             _pane = e.NameScope.Find<Panel>("PART_PaneRoot");
 
             UpdateVisualStateForDisplayMode(DisplayMode);
+            UpdatePaneStatePseudoClass(IsPaneOpen);
         }
 
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
@@ -331,19 +332,13 @@ namespace Avalonia.Controls
             else if (change.Property == IsPaneOpenProperty)
             {
                 bool isPaneOpen = change.GetNewValue<bool>();
-
+                UpdatePaneStatePseudoClass(isPaneOpen);
                 if (isPaneOpen)
                 {
-                    PseudoClasses.Add(pcOpen);
-                    PseudoClasses.Remove(pcClosed);
-
                     OnPaneOpened(new RoutedEventArgs(PaneOpenedEvent, this));
                 }
                 else
                 {
-                    PseudoClasses.Add(pcClosed);
-                    PseudoClasses.Remove(pcOpen);
-
                     OnPaneClosed(new RoutedEventArgs(PaneClosedEvent, this));
                 }
             }
@@ -579,6 +574,20 @@ namespace Avalonia.Controls
             e.Handled = true;
         }
 
+        private void UpdatePaneStatePseudoClass(bool isPaneOpen)
+        {
+            if (isPaneOpen)
+            {
+                PseudoClasses.Add(pcOpen);
+                PseudoClasses.Remove(pcClosed);
+            }
+            else
+            {
+                PseudoClasses.Add(pcClosed);
+                PseudoClasses.Remove(pcOpen);
+            }
+        }
+
         /// <summary>
         /// Coerces/validates the <see cref="IsPaneOpen"/> property value.
         /// </summary>

tests/Avalonia.Controls.UnitTests/SplitViewTests.cs

@@ -1,5 +1,7 @@
-using Avalonia.Input;
+using Avalonia.Animation;
+using Avalonia.Input;
 using Avalonia.UnitTests;
+using Moq;
 using Xunit;
 
 namespace Avalonia.Controls.UnitTests
@@ -65,7 +67,8 @@ namespace Avalonia.Controls.UnitTests
         [Fact]
         public void SplitView_TemplateSettings_Are_Correct_For_Display_Modes()
         {
-            using var app = UnitTestApplication.Start(TestServices.StyledWindow);
+            using var app = UnitTestApplication.Start(TestServices.StyledWindow
+                .With(globalClock: new MockGlobalClock()));
             var wnd = new Window
             {
                 Width = 1280,
@@ -280,5 +283,25 @@ namespace Avalonia.Controls.UnitTests
 
             Assert.True(splitView.IsPaneOpen);
         }
+
+        [Fact]
+        public void With_Default_IsPaneOpen_Value_Should_Have_Closed_Pseudo_Class_Set()
+        {
+            // Testing this control Pseudo Classes requires placing the SplitView on a window
+            // prior to asserting them, because some of the pseudo classes are set either when
+            // the template is applied or the control is attached to the visual tree
+            using var app = UnitTestApplication.Start(TestServices.StyledWindow
+                .With(globalClock: new MockGlobalClock()));
+            var wnd = new Window
+            {
+                Width = 1280,
+                Height = 720
+            };
+            var splitView = new SplitView();
+            wnd.Content = splitView;
+            wnd.Show();
+
+            Assert.Contains(splitView.Classes, ":closed".Equals);
+        }
     }
 }