Fixes T6906.
I found the code in behavior-search-typeahead.js that was throwing away the closedness-detction work done in Prejab.js::transformDatasourceResults. Modified it to re-add the correct class name to the phabricator-main-search-typeahead-result elements.
Then I found some CSS in typeahead-browse.css and completely flailed around until realizing that particular CSS only gets loaded when hitting the typeahead endpoint directly. Copied the relevant bit of CSS over to main-menu-view.css (but maybe it should be removed from typeahead-browse.css?).
This is my first JS/CSS change, so please don't assume I did anything right.