Fixes: #29006
We make the following changes:
* Do not show the typeahead menu when there are no matching topics.
* When the typeahead is not shown, pressing Tab in the topic field
should move the cursor to the compose box.
* When the typeahead is shown, pressing Tab should select the
currently focused option (as it does now), but put the cursor at
the end of the topic, rather than at the beginning.