feat: add window resizer for Ursa.

src/Ursa/Controls/Resizers/ResizeDirection.cs

@@ -0,0 +1,13 @@
+namespace Ursa.Controls;
+
+public enum ResizeDirection
+{
+    Top,
+    Bottom,
+    Left,
+    Right,
+    TopLeft,
+    TopRight,
+    BottomLeft,
+    BottomRight,
+}

src/Ursa/Controls/Resizers/WindowResizer.cs

@@ -0,0 +1,8 @@
+using Avalonia.Controls.Primitives;
+
+namespace Ursa.Controls;
+
+public class WindowResizer : TemplatedControl
+{
+    
+}

src/Ursa/Controls/Resizers/WindowResizerThumb.cs

@@ -0,0 +1,45 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
+
+namespace Ursa.Controls;
+
+public class WindowResizerThumb: Thumb
+{
+    private Window? _window;
+    
+    public static readonly StyledProperty<ResizeDirection> ResizeDirectionProperty = AvaloniaProperty.Register<WindowResizerThumb, ResizeDirection>(
+        nameof(ResizeDirection));
+
+    public ResizeDirection ResizeDirection
+    {
+        get => GetValue(ResizeDirectionProperty);
+        set => SetValue(ResizeDirectionProperty, value);
+    }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        _window = TopLevel.GetTopLevel(this) as Window;
+    }
+
+    protected override void OnPointerPressed(PointerPressedEventArgs e)
+    {
+        base.OnPointerPressed(e);
+        if (_window is null || !_window.CanResize) return;
+        var windowEdge = ResizeDirection switch
+        {
+            ResizeDirection.Top => WindowEdge.North,
+            ResizeDirection.TopRight => WindowEdge.NorthEast,
+            ResizeDirection.Right => WindowEdge.East,
+            ResizeDirection.BottomRight => WindowEdge.SouthEast,
+            ResizeDirection.Bottom => WindowEdge.South,
+            ResizeDirection.BottomLeft => WindowEdge.SouthWest,
+            ResizeDirection.Left => WindowEdge.West,
+            ResizeDirection.TopLeft => WindowEdge.NorthWest,
+            _ => throw new ArgumentOutOfRangeException()
+        };
+        _window.BeginResizeDrag(windowEdge, e);
+    }
+}

src/Ursa/Windows/UrsaWindow.cs

@@ -54,6 +54,15 @@ public class UrsaWindow: Window
         get => GetValue(IsTitleBarVisibleProperty);
         set => SetValue(IsTitleBarVisibleProperty, value);
     }
+
+    public static readonly StyledProperty<bool> IsManagedResizerVisibleProperty = AvaloniaProperty.Register<UrsaWindow, bool>(
+        nameof(IsManagedResizerVisible));
+
+    public bool IsManagedResizerVisible
+    {
+        get => GetValue(IsManagedResizerVisibleProperty);
+        set => SetValue(IsManagedResizerVisibleProperty, value);
+    }
     
     public static readonly StyledProperty<object?> TitleBarContentProperty = AvaloniaProperty.Register<UrsaWindow, object?>(
         nameof(TitleBarContent));