feat: several fixes.

src/Ursa/Controls/Shapes/PureCircle.cs

@@ -0,0 +1,45 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Media;
+
+namespace Ursa.Controls.Shapes;
+
+public class PureCircle: Control
+{
+    public static readonly StyledProperty<IBrush?> BackgroundProperty =
+        TemplatedControl.BackgroundProperty.AddOwner<PureCircle>();
+
+    public IBrush? Background
+    {
+        get => GetValue(BackgroundProperty);
+        set => SetValue(BackgroundProperty, value);
+    }
+
+    public static readonly StyledProperty<double> DiameterProperty = AvaloniaProperty.Register<PureCircle, double>(
+        nameof(Diameter));
+
+    public double Diameter
+    {
+        get => GetValue(DiameterProperty);
+        set => SetValue(DiameterProperty, value);
+    }
+
+    static PureCircle()
+    {
+        FocusableProperty.OverrideDefaultValue<PureCircle>(false);
+        AffectsMeasure<PureCircle>(DiameterProperty);
+        AffectsRender<PureCircle>(DiameterProperty, BackgroundProperty);
+    }
+
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        return new Size(Diameter, Diameter);
+    }
+
+    public override void Render(DrawingContext context)
+    {
+        double value = Diameter / 2;
+        context.DrawEllipse(Background, null, new(value, value), value, value);
+    }
+}