Add comprehensive headless tests for 10 controls with enhanced ItemsSource and method testing (#18)

* Initial plan

* Add comprehensive tests for 5 controls: Avatar, Clock, ClockTicks, AspectRatioLayout, AspectRatioLayoutItem

Co-authored-by: rabbitism <14807942+rabbitism@users.noreply.github.com>

* Complete comprehensive headless tests for all 10 controls - added 72 more tests

Co-authored-by: rabbitism <14807942+rabbitism@users.noreply.github.com>

* Add requested test cases for MultiComboBox and TreeComboBox - ItemsSource, Remove, Clear, hierarchical data

Co-authored-by: rabbitism <14807942+rabbitism@users.noreply.github.com>

* test: open dropdown to trigger materialization.

* test: set CurrentCulture to en-US for calendar view tests

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: rabbitism <14807942+rabbitism@users.noreply.github.com>
Co-authored-by: rabbitism <dongbin@irihi.tech>

tests/HeadlessTest.Ursa/Controls/MultiComboBoxItemTests/MultiComboBoxItemTests.cs

@@ -0,0 +1,200 @@
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using UrsaControls = Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls.MultiComboBoxItemTests;
+
+public class MultiComboBoxItemTests
+{
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Initialize_With_Default_Values()
+    {
+        // Arrange & Act
+        var item = new UrsaControls.MultiComboBoxItem();
+
+        // Assert
+        Assert.False(item.IsSelected);
+        Assert.Null(item.Content);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Set_IsSelected_Property()
+    {
+        // Arrange
+        var window = new Window();
+        var item = new UrsaControls.MultiComboBoxItem();
+        window.Content = item;
+        window.Show();
+
+        // Act
+        item.IsSelected = true;
+
+        // Assert
+        Assert.True(item.IsSelected);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Toggle_IsSelected_Property()
+    {
+        // Arrange
+        var window = new Window();
+        var item = new UrsaControls.MultiComboBoxItem();
+        window.Content = item;
+        window.Show();
+
+        // Act
+        item.IsSelected = true;
+        item.IsSelected = false;
+
+        // Assert
+        Assert.False(item.IsSelected);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Set_Content_Property()
+    {
+        // Arrange
+        var window = new Window();
+        var item = new UrsaControls.MultiComboBoxItem();
+        var content = "Test Content";
+        window.Content = item;
+        window.Show();
+
+        // Act
+        item.Content = content;
+
+        // Assert
+        Assert.Equal(content, item.Content);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Set_Content_As_Control()
+    {
+        // Arrange
+        var window = new Window();
+        var item = new UrsaControls.MultiComboBoxItem();
+        var content = new Button { Content = "Button Content" };
+        window.Content = item;
+        window.Show();
+
+        // Act
+        item.Content = content;
+
+        // Assert
+        Assert.Equal(content, item.Content);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Be_Visible_When_Added_To_Window()
+    {
+        // Arrange
+        var window = new Window();
+        var item = new UrsaControls.MultiComboBoxItem();
+
+        // Act
+        window.Content = item;
+        window.Show();
+
+        // Assert
+        Assert.True(item.IsVisible);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Inherit_From_ContentControl()
+    {
+        // Arrange & Act
+        var item = new UrsaControls.MultiComboBoxItem();
+
+        // Assert
+        Assert.IsAssignableFrom<ContentControl>(item);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Be_Focusable()
+    {
+        // Arrange & Act
+        var item = new UrsaControls.MultiComboBoxItem();
+
+        // Assert
+        Assert.True(item.Focusable);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Handle_Null_Content()
+    {
+        // Arrange
+        var window = new Window();
+        var item = new UrsaControls.MultiComboBoxItem();
+        window.Content = item;
+        window.Show();
+
+        // Act
+        item.Content = "Test";
+        item.Content = null;
+
+        // Assert
+        Assert.Null(item.Content);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Handle_DataContext()
+    {
+        // Arrange
+        var window = new Window();
+        var item = new UrsaControls.MultiComboBoxItem();
+        var dataContext = new { Name = "Test", Value = 42 };
+        window.Content = item;
+        window.Show();
+
+        // Act
+        item.DataContext = dataContext;
+
+        // Assert
+        Assert.Equal(dataContext, item.DataContext);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Support_Multiple_Selection_States()
+    {
+        // Arrange
+        var window = new Window();
+        var item1 = new UrsaControls.MultiComboBoxItem { Content = "Item 1" };
+        var item2 = new UrsaControls.MultiComboBoxItem { Content = "Item 2" };
+        var stackPanel = new StackPanel();
+        stackPanel.Children.Add(item1);
+        stackPanel.Children.Add(item2);
+        window.Content = stackPanel;
+        window.Show();
+
+        // Act
+        item1.IsSelected = true;
+        item2.IsSelected = true;
+
+        // Assert
+        Assert.True(item1.IsSelected);
+        Assert.True(item2.IsSelected);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxItem_Should_Change_Selection_Independently()
+    {
+        // Arrange
+        var window = new Window();
+        var item1 = new UrsaControls.MultiComboBoxItem { Content = "Item 1" };
+        var item2 = new UrsaControls.MultiComboBoxItem { Content = "Item 2" };
+        var stackPanel = new StackPanel();
+        stackPanel.Children.Add(item1);
+        stackPanel.Children.Add(item2);
+        window.Content = stackPanel;
+        window.Show();
+
+        // Act
+        item1.IsSelected = true;
+        item2.IsSelected = true;
+        item1.IsSelected = false;
+
+        // Assert
+        Assert.False(item1.IsSelected);
+        Assert.True(item2.IsSelected);
+    }
+}