The element with role button has aria-haspopup set to either menu or true.
When the menu is displayed, the element with role button has aria-expanded set to true. When the menu is hidden, it is recommended that aria-expanded is not present. If aria-expanded is specified when the menu is hidden, it is set to false.
The element that contains the menu items displayed by activating the button has role menu.
Optionally, the element with role button has a value specified for aria-controls that refers to the element with role menu.
With focus on the button, “Enter” or “Space” opens the menu and places focus on the first menu item. Up and down arrows move focus through menu items.
