fix calculation of working area.

native/Avalonia.Native/src/OSX/Screens.mm

@@ -4,6 +4,14 @@ class Screens : public ComSingleObject<IAvnScreens, &IID_IAvnScreens>
 {
     public:
     FORWARD_IUNKNOWN()
+    
+    private:
+    CGFloat PrimaryDisplayHeight()
+    {
+      return NSMaxY([[[NSScreen screens] firstObject] frame]);
+    }
+    
+public:
     virtual HRESULT GetScreenCount (int* ret) override
     {
         @autoreleasepool
@@ -25,15 +33,15 @@ class Screens : public ComSingleObject<IAvnScreens, &IID_IAvnScreens>
             
             auto screen = [[NSScreen screens] objectAtIndex:index];
             
-            ret->Bounds.X = [screen frame].origin.x;
-            ret->Bounds.Y = [screen frame].origin.y;
             ret->Bounds.Height = [screen frame].size.height;
             ret->Bounds.Width = [screen frame].size.width;
+            ret->Bounds.X = [screen frame].origin.x;
+            ret->Bounds.Y = PrimaryDisplayHeight() - [screen frame].origin.y - ret->Bounds.Height;
             
-            ret->WorkingArea.X = [screen visibleFrame].origin.x;
-            ret->WorkingArea.Y = [screen visibleFrame].origin.y;
             ret->WorkingArea.Height = [screen visibleFrame].size.height;
             ret->WorkingArea.Width = [screen visibleFrame].size.width;
+            ret->WorkingArea.X = [screen visibleFrame].origin.x;
+            ret->WorkingArea.Y = ret->Bounds.Height - [screen visibleFrame].origin.y - ret->WorkingArea.Height;
             
             ret->PixelDensity = [screen backingScaleFactor];