feat: deal with virtualization.

src/Ursa/Controls/ComboBox/MultiComboBoxItem.cs

@@ -1,11 +1,28 @@
 using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Controls.Mixins;
+using Avalonia.Input;
 using Irihi.Avalonia.Shared.Helpers;
 
 namespace Ursa.Controls;
 
-public class MultiComboBoxItem: ListBoxItem
+public class MultiComboBoxItem: ContentControl
 {
+    public static readonly StyledProperty<bool> IsSelectedProperty = AvaloniaProperty.Register<MultiComboBoxItem, bool>(
+        nameof(IsSelected));
+
+    public bool IsSelected
+    {
+        get => GetValue(IsSelectedProperty);
+        set => SetValue(IsSelectedProperty, value);
+    }
+    
+    static MultiComboBoxItem()
+    {
+        IsSelectedProperty.AffectsPseudoClass<MultiComboBoxItem>(":selected");
+        PressedMixin.Attach<MultiComboBoxItem>();
+        FocusableProperty.OverrideDefaultValue<MultiComboBoxItem>(true);
+    }
     public MultiComboBoxItem()
     {
         this.GetObservable(IsFocusedProperty).Subscribe(a=> {
@@ -15,4 +32,18 @@ public class MultiComboBoxItem: ListBoxItem
             }
         });
     }
+    
+    protected override void OnPointerPressed(PointerPressedEventArgs e)
+    {
+        base.OnPointerPressed(e);
+        if (e.Handled)
+        {
+            return;
+        }
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+        {
+            this.IsSelected = !this.IsSelected;
+            e.Handled = true;
+        }
+    }
 }