Passed to MentionDropdown by TextEditor, from the state of
useMentionAutocomplete.
id: string
The textarea's aria-controls. Option i gets the id ${id}-${i}, which
the textarea names as its aria-activedescendant.
selectedIndex: number
onSelect: (username: string) => void
optional
style: CSSProperties
From useAnchoredFloat, or hidden until it has placed the list.