Menus not opening on drag-and-drop

Yes, the button is outlined in exactly the same way as when I mouse-over it normally. Also if I drag to a submenu of a menu that is already open, the submenu is highlighted just as it is when I mouse-over it without dragging anything. But the menus and submenus don't open to allow me to drop the item within them.

I can still successfully drag-and-drop an item onto the toolbar itself, or from a menu to its parent menus or to explorer - just not to a menu that isn't already open.