fix osx popup positioning.

src/Avalonia.Native/AvaloniaNativePlatform.cs

@@ -1,7 +1,6 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 using System;
-using System.Diagnostics.Contracts;
 using System.Runtime.InteropServices;
 using Avalonia.Controls.Platform;
 using Avalonia.Input;

src/Avalonia.Native/OsxManagedPopupPositionerPopupImplHelper.cs

@@ -0,0 +1,18 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+using Avalonia.Controls.Primitives.PopupPositioning;
+using Avalonia.Platform;
+
+namespace Avalonia.Native
+{
+    class OsxManagedPopupPositionerPopupImplHelper : ManagedPopupPositionerPopupImplHelper
+    {
+        public OsxManagedPopupPositionerPopupImplHelper(IWindowBaseImpl parent, MoveResizeDelegate moveResize) : base(parent, moveResize)
+        {
+
+        }
+        public override Point TranslatePoint(Point pt) => pt;
+
+        public override Size TranslateSize(Size size) => size;
+    }
+}

src/Avalonia.Native/PopupImpl.cs

@@ -22,7 +22,7 @@ namespace Avalonia.Native
             {
                 Init(factory.CreatePopup(e), factory.CreateScreens());
             }
-            PopupPositioner = new ManagedPopupPositioner(new ManagedPopupPositionerPopupImplHelper(parent, MoveResize));
+            PopupPositioner = new ManagedPopupPositioner(new OsxManagedPopupPositionerPopupImplHelper(parent, MoveResize));
         }
 
         private void MoveResize(PixelPoint position, Size size, double scaling)