feat: add window thumb class.

src/Ursa/Controls/TitleBar/WindowThumb.cs

@@ -0,0 +1,51 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+using Avalonia.Media;
+
+namespace Ursa.Controls;
+
+public class WindowThumb: Control
+{
+    public static readonly StyledProperty<IBrush?> BackgroundProperty = AvaloniaProperty.Register<WindowThumb, IBrush?>(
+        nameof(Background), Brushes.Transparent);
+
+    public IBrush? Background
+    {
+        get => GetValue(BackgroundProperty);
+        set => SetValue(BackgroundProperty, value);
+    }
+    
+    static WindowThumb()
+    {
+        IsHitTestVisibleProperty.OverrideDefaultValue<WindowThumb>(true);
+        HorizontalAlignmentProperty.OverrideDefaultValue<WindowThumb>(Avalonia.Layout.HorizontalAlignment.Stretch);
+        VerticalAlignmentProperty.OverrideDefaultValue<WindowThumb>(Avalonia.Layout.VerticalAlignment.Stretch);
+    }
+
+    public WindowThumb()
+    {
+        this.AddHandler(PointerPressedEvent, OnPressed, RoutingStrategies.Direct);
+        this.AddHandler(DoubleTappedEvent, OnTapped, RoutingStrategies.Direct);
+    }
+
+    private void OnTapped(object sender, TappedEventArgs e)
+    {
+        if (this.VisualRoot is not Window visualRoot) return;
+        if (!visualRoot.CanResize) return;
+        if ( visualRoot.WindowState == WindowState.FullScreen) return;
+        visualRoot.WindowState = visualRoot.WindowState == WindowState.Maximized ? WindowState.Normal : WindowState.Maximized;
+    }
+
+    private void OnPressed(object sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+        {
+            if (VisualRoot is Window window)
+            {
+                window.BeginMoveDrag(e);
+            }
+        }
+    }
+}