I haven't found any information speaking to the following scenario. I read through https://w3c.github.io/aria/#managingfocus_useragents and I don't think it addresses this question, but the language is a little hard to follow so please let me know if I'm just missing something :)

Example:

Suppose I have a role=combobox widget with a displayed role=listbox#some-list-id for autocomplete. Suppose also that the listbox widget uses aria-activedescendant for managing focus, i.e. the role=listbox element has tabindex=0 but none of its role=option children are focusable.

<input role="combobox" aria-haspopup="listbox" ... aria-activedescendant="some-list-id" />
<div role="listbox" id="some-list-id" aria-activedescendant="option-id-3">
  <div role="option" id="option-id-1">One</div>
  <div role="option" id="option-id-2">Two</div>
  <div role="option" id="option-id-3">Three</div> <!-- I would expect the screen reader to understand this option is "focused" if DOM focus is on the input element -->
</div>

In this situation, I feel the text field in the combobox ought to be able to set its aria-activedescendant="some-list-id", i.e. it doesn't need to know about the list's currently active item -- the combobox can just point at the listbox, and the listbox can then point to one of its internal options as active.