swap jQuery's `trigger(...)` with the more verbose native `dispatchEvent(...)`, as the former may not always behave/bubble correctly (observed while trying to write unit tests for keyboard handling of ARIA tab navigation), which may lead to this test passing even though it fails in real usage.

Add unit test from #28912 to v5

Just firing the focus event isn't enough ... need to actually call the proper `focus()` method for it to actually do it...