Dropdown Menu: Arrow keys do not select items after triggering the menu with the mouse

[rak-phillip]

Dropdown menu items are not able to be focused via keyboard after opening a dropdown menu with the mouse.

Steps to reproduce

1. Open a dropdown menu with the mouse (page header actions, user menu)
2. press the down key

Expected behavior

The first dropdown item will receive focus

Actual behavior

No dropdown items receive focus

Acceptance criteria

• Pressing the down key after opening a dropdown with the mouse focuses the first item
• Subsequent up/down keypresses focus the appropriate item

Details

It looks like handleKeydown inadvertently got deleted. This causes a regression where arrow keys can no longer handle selection after opening a dropdown with the mouse. I suspect this got deleted due to a bad resolution for a merge conflict.

Originally posted by @rak-phillip in #13357 (comment)

[a-arias]

Existing tests are located in: #13357,

the coverage looks good moving to done ✅