Drag and Drop in a QTreeWidget subclass

I have two types of items (say A & B) in a tree widget. A button will separate these items.
When I drop items of type A (which are present at the top of the Button), on to items of type B (which are present below the button), the type A items will be added to type B items as child items.

Once all the type A items are dropped on to type B items, the Button will be moved to the top of the list.

My requirement is when I drop type A items again on to the Button, they should be moved to the top of the list.

But the problem here is, the drop event function is not getting called when I drop the items on to Button.

This works fine on Windows. But on Mac this is not working!!

Any ideas on how to implement this?

Drop event method of which widget is not called? The button or the tree widget?

Button as well as the tree widget also!