Fix window position on macOS

src/Utilities.cs

@@ -3,6 +3,7 @@ using System.IO;
 using System.Runtime.InteropServices;
 
 using Avalonia;
+using Avalonia.Controls;
 using Avalonia.Media.Imaging;
 using Avalonia.Platform;
 
@@ -20,6 +21,17 @@ namespace AvaloniaCoreRTDemo
             return new Bitmap(assetStream);
         }
 
+        public static PixelPoint GetWindowPosition(Window window)
+        {
+            if (!IsOSX || !window.FrameSize.HasValue)
+                return window.Position;
+            else
+            {
+                Int32 yOffset = (Int32)(window.FrameSize.Value.Height - window.ClientSize.Height);
+                return new(window.Position.X, window.Position.Y + yOffset);
+            }
+        }
+
         public static Bitmap GetImageFromFile(String path)
         {
             try

src/Windows/MainWindow.axaml.cs

@@ -25,7 +25,7 @@ namespace AvaloniaCoreRTDemo.Windows
 
         IThemeSwitch IMainWindow.ThemeSwitch => (IThemeSwitch)this._app!;
         IMainWindowState IMainWindow.Model => (IMainWindowState)this.MainControl.DataContext!;
-        PixelPoint IMainWindow.Position => this.Position;
+        PixelPoint IMainWindow.Position => Utilities.GetWindowPosition(this);
         Size IMainWindow.ClientSize => this.ClientSize;
         Size? IMainWindow.FrameSize => this.FrameSize;
         WindowState IMainWindow.State => this.WindowState;