[Accessibility] Don't prepopulate aria-activedescendant with a pointer to a hidden list #1512
Description
Due to some changes in Firefox, a bug was uncovered that both earlier versions of Firefox and current versions of Chrome are masking. When initially constructing the text input, after #129 was fixed, aria-activedescendant already points to the ul with ID ´compose-autosuggest-list-home`. That UL, since nothing has been typed yet, or when no suggestions are available, is inside a div that is aria-hidden="true", and thus also hidden. Firefox is, however, due to a bug I still need to file, creating an accessible for it for some reason now. That accessible is now masking the input field. The result is that I am not able to see what I am typing, and focus is inside a list that is essentially not there.
To fix, it would be best if aria-activedescendant was not set initially, and only gets set if the list is visible and a list item actually needs to be focused due to it being an autocomplete result. If the autocomplete goes away, and the list is hidden, aria-activedescendant should be cleared again. Meaning the attribute be removed.