The dropdown component's tab and shift+tab handling violate ARIA practices for menuitems

[brichwin]

Describe the bug

When a menu item in an open IU Rivet dropdown menu has focus, pressing tab and shift+tab is moving to the next/previous menu item, optionally wrapping from the last to first or first to last. This behavior does not follow the ARIA practices for role="menuitem".

Note: Failing to follow ARIA practices reduces a control to "mystery meat navigation" because they do not follow a users established mental model for how a menu/menuitem should behave.

Expected behavior

Under the ARIA practices for keyboard interaction for a menu or menu bar, it states:

Tab and Shift+Tab:
...When focus is on a menuitem in a menu or menubar, move focus out of the menu or menubar, and close all menus and submenus.

Note: When handling Tab and Shift+Tab when a menuitem has focus, closing the menu is the same as canceling. The focused menu item is not activated.

Relevant context (required)

• Please see the Actions Menu Button Example Using element.focus() on the W3C.github.io/aria-practices site for a working example that meets the mental model Assistive Technology and keyboard only users will be expecting.
• For a failing example, please see Rivet 1.8.2 Dropdown example
• In case it helps, here is a Kaltura video demonstrating incorrect behavior for tab and shift+tab by menu items in an open IU Rivet dropdown component's menu vs. the expected behavior.

Steps to reproduce

1. Navigate to Rivet 1.8.2 Dropdown example.
2. Move keyboard focus to the "Navigation menu" dropdown button.
3. Press one of [enter, space, down arrow]
   to open the menu.
4. In cases where no menuitem has focus (due to a different bug), press the down arrow key to focus a menu item.
5. Press tab and/or shift+tab as desired. Note that the result is moving between menu items instead of closing the menu.

[brichwin]

This issue is present in Rivet 1 and Rivet 2. Since the issue was discovered by a Canvas user, please address the Rivet 1 codebase as well.