feat:

1. fix #427.
2. add headless test for this fix.

tests/HeadlessTest.Ursa/App.axaml

@@ -0,0 +1,10 @@
+<Application xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:semi="https://irihi.tech/semi"
+             xmlns:u-semi="https://irihi.tech/ursa/themes/semi"
+             x:Class="HeadlessTest.Ursa.App">
+    <Application.Styles>
+        <semi:SemiTheme />
+        <u-semi:SemiTheme/>
+    </Application.Styles>
+</Application>

tests/HeadlessTest.Ursa/App.axaml.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace HeadlessTest.Ursa;
+
+public partial class App : Application
+{
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+}

tests/HeadlessTest.Ursa/Controls/BreadcrumbTests/BreadcrumbTests.cs

@@ -0,0 +1,66 @@
+using System.Collections.ObjectModel;
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using Avalonia.LogicalTree;
+using Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls;
+
+public class BreadcrumbTests
+{
+    [AvaloniaFact]
+    public void BreadcrumbItem_Should_Have_Correct_PseudoClasses()
+    {
+        var window = new Window();
+        var breadcrumb = new Breadcrumb();
+        window.Content = breadcrumb;
+        window.Show();
+        var item1 = new BreadcrumbItem();
+        var item2 = new BreadcrumbItem();
+        breadcrumb.Items.Add(item1);
+        Assert.Contains(":last", item1.Classes);
+        breadcrumb.Items.Add(item2);
+        Assert.Contains(":last", item2.Classes);
+        Assert.DoesNotContain(":last", item1.Classes);
+    }
+    
+    [AvaloniaFact]
+    public void Generated_BreadcrumbItem_Should_Have_Correct_PseudoClasses()
+    {
+        var window = new Window();
+        var breadcrumb = new Breadcrumb();
+        window.Content = breadcrumb;
+        window.Show();
+        var item1 = new TextBlock();
+        var item2 = new TextBlock();
+        breadcrumb.Items.Add(item1);
+        var firstItem = breadcrumb.GetLogicalChildren().OfType<BreadcrumbItem>().FirstOrDefault();
+        Assert.NotNull(firstItem);
+        Assert.Contains(":last", firstItem.Classes);
+        breadcrumb.Items.Add(item2);
+        var lastItem = breadcrumb.GetLogicalChildren().OfType<BreadcrumbItem>().LastOrDefault();
+        Assert.NotNull(lastItem);
+        Assert.Contains(":last", lastItem.Classes);
+        Assert.DoesNotContain(":last", firstItem.Classes);
+    }
+    
+    [AvaloniaFact]
+    public void BreadcrumbItem_FromItemsSource_Should_Have_Correct_PseudoClasses()
+    {
+        var window = new Window();
+        var breadcrumb = new Breadcrumb();
+        window.Content = breadcrumb;
+        window.Show();
+        var items = new ObservableCollection<string>();
+        breadcrumb.ItemsSource = items;
+        items.Add("Item 1");
+        var item1 = breadcrumb.GetLogicalChildren().OfType<BreadcrumbItem>().FirstOrDefault();
+        Assert.NotNull(item1);
+        Assert.Contains(":last", item1.Classes);
+        items.Add("Item 2");
+        var item2 = breadcrumb.GetLogicalChildren().OfType<BreadcrumbItem>().LastOrDefault();
+        Assert.NotNull(item2);
+        Assert.Contains(":last", item2.Classes);
+        Assert.DoesNotContain(":last", item1.Classes);
+    }
+}

tests/HeadlessTest.Ursa/HeadlessTest.Ursa.csproj

@@ -0,0 +1,30 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net8.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="Avalonia.Headless.XUnit" Version="11.1.3" />
+        <PackageReference Include="coverlet.collector" Version="6.0.0"/>
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0"/>
+        <PackageReference Include="Semi.Avalonia" Version="11.1.0.4" />
+        <PackageReference Include="xunit" Version="2.5.3"/>
+        <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <Using Include="Xunit"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\..\src\Ursa\Ursa.csproj" />
+        <ProjectReference Include="..\..\src\Ursa.Themes.Semi\Ursa.Themes.Semi.csproj" />
+    </ItemGroup>
+
+</Project>

tests/HeadlessTest.Ursa/TestAppBuilder.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Headless;
+using HeadlessTest.Ursa;
+
+[assembly: AvaloniaTestApplication(typeof(TestAppBuilder))]
+
+namespace HeadlessTest.Ursa;
+
+public class TestAppBuilder
+{
+    public static AppBuilder BuildAvaloniaApp() =>
+        AppBuilder.Configure<App>().UseHeadless(new AvaloniaHeadlessPlatformOptions());
+}

tests/Test.Ursa/Test.Ursa.csproj

@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net8.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="coverlet.collector" Version="6.0.0"/>
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0"/>
+        <PackageReference Include="xunit" Version="2.5.3"/>
+        <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <Using Include="Xunit"/>
+    </ItemGroup>
+
+    <ItemGroup>
+      <ProjectReference Include="..\..\src\Ursa\Ursa.csproj" />
+    </ItemGroup>
+
+</Project>