For the Dropdown Chooser control's "closed" state, show the
current value of the control in either a button or a text
field. To its right, put a down arrow. This may be in its
own button, or not, as you see fit; experiment and see what
looks good and makes sense to your users. A click on the
arrow (or the whole control) brings up the chooser panel,
and a second click closes it again.
Tailor the chooser panel for the choice the user needs to make.
Make it relatively small and compact; its visual organization
should be a familiar information-graphics format, such as a
list, a table, an outline-type tree, or a specialized format
like a calendar or calculator (see the following examples).
Scrolling the panel is OK if the user understands that
it's a choice from a large set, such as a file from a
filesystem, but keep in mind that scrolling one of these popup
panels is not easy for people without perfect dexterity!
Links or buttons on the panel can in turn bring up secondary UIs
-- for example, color-chooser dialogs, file-finder dialogs, or
help pages -- that help the user choose a value. These are
usually modal dialog boxes. In fact, if you intend to use one of
these modal dialogs as the primary way the user picks a value
(say, by launching it from a button), you could use a Dropdown
Chooser instead of going straight to the modal dialog. The pop-up
panel could contain the most common or recently chosen items. By
making frequently chosen items so easy to pick, you reduce the
total time (or number of clicks) that it takes for an average user
to pick values.