Mainwindow: Toolbar allows docking widgets from outside the window area

QMainWindow's toolbars allow re-docking floating toolbars from anywhere left of (for horizontal areas) or above (for vertical areas) the actual docking area. See screenshot, where the floating toolbar being dragged would now be attached in the blue higlighted space when dropped:

This is especially irritating when working with two screens; moving tools to the other screen will randomly re-dock them.