[OSX] fix popup position and mouse device positioning.

src/Avalonia.Native/AvaloniaNativePlatform.cs

@@ -21,7 +21,6 @@ namespace Avalonia.Native
         [DllImport("libAvaloniaNative")]
         static extern IntPtr CreateAvaloniaNative();
 
-        internal static readonly MouseDevice MouseDevice = new MouseDevice();
         internal static readonly KeyboardDevice KeyboardDevice = new KeyboardDevice();
 
         public Size DoubleClickSize => new Size(4, 4);

src/Avalonia.Native/WindowImplBase.cs

@@ -96,7 +96,7 @@ namespace Avalonia.Native
         public Action<Rect> Paint { get; set; }
         public Action<Size> Resized { get; set; }
         public Action Closed { get; set; }
-        public IMouseDevice MouseDevice => AvaloniaNativePlatform.MouseDevice;
+        public IMouseDevice MouseDevice => _mouse;
         public abstract IPopupImpl CreatePopup();