test: mvvm tests.

tests/HeadlessTest.Ursa/Controls/AnchorTests/TestView2.axaml.cs

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+using System.Collections.ObjectModel;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace HeadlessTest.Ursa.Controls.AnchorTests;
+
+public partial class TestView2 : UserControl
+{
+    public TestView2()
+    {
+        InitializeComponent();
+        DataContext = new AnchorDemoViewModel();
+    }
+}
+
+public partial class AnchorDemoViewModel: ObservableObject
+{
+    public List<AnchorItemViewModel> AnchorItems { get; } = new()
+    {
+        new AnchorItemViewModel { AnchorId = "anchor1", Header = "Anchor 1" },
+        new AnchorItemViewModel { AnchorId = "anchor2", Header = "Anchor 2" },
+        new AnchorItemViewModel
+        {
+            AnchorId = "anchor3", Header = "Anchor 3",
+            Children =
+            [
+                new AnchorItemViewModel() { AnchorId = "anchor3-1", Header = "Anchor 3.1" },
+                new AnchorItemViewModel() { AnchorId = "anchor3-2", Header = "Anchor 3.2" },
+                new AnchorItemViewModel() { AnchorId = "anchor3-3", Header = "Anchor 3.3" }
+            ]
+        },
+        new AnchorItemViewModel { AnchorId = "anchor4", Header = "Anchor 4" },
+        new AnchorItemViewModel { AnchorId = "anchor5", Header = "Anchor 5" },
+        new AnchorItemViewModel { AnchorId = "anchor6", Header = "Anchor 6" },
+        new AnchorItemViewModel { AnchorId = "anchor7", Header = "Anchor 7" },
+    };
+}
+
+public partial class AnchorItemViewModel: ObservableObject
+{
+    [ObservableProperty] private string? _anchorId;
+    [ObservableProperty] private string? _header;
+    public ObservableCollection<AnchorItemViewModel>? Children { get; set; }
+}