confuse with treeview’s mouse click event and drag event

Using Gtk::TreeView in my code, get confused with the mouse click event and drag event.
What I am expected is that: press the mouse, begin the drag event, release the mouse begin click event.
But I get this: press the mouse, begin the click event, then the drag event.
So there is a confusion to user: while he select a item, and then drag the item to another widget, the item unselected!
and if the selection mode is multiple selection, the confusion will get increase.

I do think this is a bug in gtkmm or gtk+, what’s you option?