how do you create a popdown menu like the one in fuzzyfinder?

When you launch FuzzyFinder, a 1 row window appears at the top along with a drop down list of matches, which you can go up and down with C-P and C-N. I know how to generate the 1-row mini-window, but how does one generate the drop down list with VimScript?

It looks like this type of drop down list is also used for code completions in V开发者_如何学运维im.

This is a big and complex set of interrelated functionality in Vim, coming under umbrella of 'completion methods'. I think FuzzyFinder uses mostly "completion functions". In any case you can start reading about it in Vim help by doing:

:h complete-functions

and/or

:h popupmenu-completion