SelectField and the refresh-interval picker are built on a new Listbox, a
select-only combobox: a button that keeps focus and a listbox in the top
layer, so it works the same in every current browser, is not clipped by a
scrolling ancestor and shows above a modal dialog. Keys, type-ahead and
placement are pure functions with tests. required becomes aria-required, as a
button has no native form validation.
