child pane drag // drop not
 

In child pane user tries to drag / drop one item onto another, i.e. move to create a grand child item.

Grandchild item is not created. Instead a copy of the dragged item is created at the same level.

By design, dragging within the Related Items pane creates a duplicate sibling item.

OK but does this make sense? Why not allow user to manipulate items in the child pane? Sometimes it's more convenient this way especially when importing data w same subject from different sources. Maybe you would consider adding it to the todo list ??