I'm noticing some discrepancy in the Combobox interaction pattern guidelines.

In this document: https://www.w3.org/WAI/ARIA/apg/patterns/combobox/#keyboard_interaction, in the "Combobox Keyboard Interaction" section, it states:

When focus is in the combobox... Up Arrow (Optional): If the popup is available, places focus on the last focusable element in the popup.

However, in the following section, "Listbox Popup Keyboard Interaction", it states:

When focus is in a listbox popup... Down Arrow: Moves focus to and selects the next option. If focus is on the last option, either returns focus to the combobox or does nothing.

and

Up Arrow: Moves focus to and selects the previous option. If focus is on the first option, either returns focus to the combobox or does nothing.

This raises the concern: if a user presses the Up Arrow to move focus to the last element in the popup, this action may not be reversible, except by pressing the Up Arrow until they reach the top of the menu.

Finally, this demo: Editable Combobox With List Autocomplete Example
behaves counter to the "Listbox Popup Keyboard Interaction" guidelines—it "rolls over" focus from first to last, and vice-versa.