Quote Originally Posted by torvista View Post
The necessary javascript file is:
instant_search_dropdown.min.js

This is loaded by the file:
auto.instant_search_observer.php

and triggered from the notifier:
'NOTIFY_FOOTER_END'

in tpl_main_page.php

So, something is broken/missing in that chain of events.
Nice detective work, @torvista. I'm guessing that that notifier is missing, although that's been "in-core" since early 2016.