Dropdown component

• I have searched the issues of this repository and believe that this is not a duplicate.

Stemming from discussion in this bug, it would be great to have a "Dropdown" component. I see it as a sort of non-modal Popover--it closes in response to clicks outside, but doesn't swallow those clicks, so that you don't have to click twice to activate another interactive element on the page (once to dismiss the open Popover, then again to activate the other element).

My specific use case is that I have a pair of icons next to each other in an AppBar, that each open a small dropdown menu. I would like users to be able to switch between menus (or go to an input/whatever else on the page) without having to click twice.

For reference, I am converting a project of mine from reactstrap to Material UI, and previously I was using reactstrap's Dropdown component, which behaves how I would like this to behave. @oliviertassinari linked me to https://material.io/guidelines/components/buttons.html#buttons-dropdown-buttons as the Material UI spec that seems to specify this case.

Review issue #9893 and the linked discussion in bug #8202, then compare the requested non-modal outside-click behavior with the Material Design dropdown-buttons specification and the referenced Reactstrap Dropdown. Done means defining and implementing a Dropdown component that lets users switch between menus or activate other page controls without a second click.