When a drop occurs, the model index corresponding to the parent item will either be valid, indicating that the drop occurred on an item, or it will be invalid, indicating that the drop occurred somewhere in the view that corresponds to top level of the model.

We initially examine the row number supplied to see if we can use it to insert items into the model, regardless of whether the parent index is valid or not.

If the parent model index is valid, the drop occurred on an item. In this simple list model, we find out the row number of the item and use that value to insert dropped items into the top level of the model.