Defined in carb/simplegui/SimpleGuiTypes.h

const DragDropFlags carb::simplegui::kDragDropFlagSourceAllowNullID = 1 << 3

Allow items such as Text(), Image() that have no unique identifier to be used as drag source, by manufacturing a temporary identifier based on their window-relative position. This is extremely unusual within the simplegui ecosystem and so we made it explicit.