One out of the completion tooltip, pop-up frame, or menu can be displayed automatically when you start completing (it makes no sense to display more than one of them at a time; they would simply mask each other). It can either be displayed immediately, or only after Emacs has been idle for a number of seconds. The following variables control this feature:
tooltip, pop-up or menu, the
corresponding list of completions is displayed automatically when
completing. When nil, nothing is displayed automatically (they can still
be displayed manually when required).