[OSX] borderless window can transistion from FullScreen to Maximised correctly.

native/Avalonia.Native/src/OSX/window.h

@@ -37,6 +37,8 @@ struct IWindowStateChanged
     virtual void WindowStateChanged () = 0;
     virtual void StartStateTransition () = 0;
     virtual void EndStateTransition () = 0;
+    virtual SystemDecorations Decorations () = 0;
+    virtual AvnWindowState WindowState () = 0;
 };
 
 #endif /* window_h */

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

@@ -499,6 +499,16 @@ private:
         _transitioningWindowState = false;
     }
     
+    SystemDecorations Decorations () override
+    {
+        return _decorations;
+    }
+    
+    AvnWindowState WindowState () override
+    {
+        return _lastWindowState;
+    }
+    
     void WindowStateChanged () override
     {
         if(!_inSetWindowState && !_transitioningWindowState)
@@ -1587,6 +1597,13 @@ NSArray* AllLoopModes = [NSArray arrayWithObjects: NSDefaultRunLoopMode, NSEvent
     if(parent != nullptr)
     {
         parent->EndStateTransition();
+        
+        if(parent->Decorations() != SystemDecorationsFull && parent->WindowState() == Maximized)
+        {
+            NSRect screenRect = [[self screen] visibleFrame];
+            [self setFrame:screenRect display:YES];
+        }
+        
         parent->WindowStateChanged();
     }
 }