feat: simple AvatarGroupPanel.

src/Ursa/Controls/Avatar/AvatarGroup.cs

@@ -1,7 +1,6 @@
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.Templates;
-using Avalonia.Layout;
 
 namespace Ursa.Controls;
 
@@ -10,7 +9,7 @@ public class AvatarGroup : ItemsControl
     public const string PART_RenderMore = "PART_RenderMore";
 
     private static readonly FuncTemplate<Panel?> DefaultPanel =
-        new(() => new StackPanel { Orientation = Orientation.Horizontal });
+        new(() => new AvatarGroupPanel());
 
     public static readonly StyledProperty<int> MaxCountProperty = AvaloniaProperty.Register<AvatarGroup, int>(
         nameof(MaxCount));

src/Ursa/Controls/Avatar/AvatarGroupPanel.cs

@@ -0,0 +1,46 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+
+namespace Ursa.Controls;
+
+public class AvatarGroupPanel : Panel
+{
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        Size size = new Size();
+        availableSize = availableSize.WithWidth(double.PositiveInfinity);
+        var children = Children;
+        foreach (var child in children)
+        {
+            child.Measure(availableSize);
+            Size desiredSize = child.DesiredSize;
+            size = size.WithWidth(size.Width + desiredSize.Width);
+            size = size.WithHeight(Math.Max(size.Height, desiredSize.Height));
+        }
+
+        size = size.WithWidth(size.Width);
+        return size;
+    }
+
+    protected override Size ArrangeOverride(Size finalSize)
+    {
+        Rect rect = new Rect(finalSize);
+        double num = 0d;
+        var children = Children;
+        foreach (var child in children)
+        {
+            Size desiredSize = child.DesiredSize;
+            double width = desiredSize.Width;
+            double height = Math.Max(desiredSize.Height, finalSize.Height);
+            rect = rect.WithX(rect.X + num);
+            rect = rect.WithWidth(width);
+            rect = rect.WithHeight(height);
+            num = width;
+            child.Arrange(rect);
+        }
+
+        RaiseEvent(new RoutedEventArgs(StackPanel.HorizontalSnapPointsChangedEvent));
+        return finalSize;
+    }
+}