Enable opening submenu from search results with just the keyboard

The keyboard shortcut help mentions that you can use `→` and `←` to show/hide sublists in the sidebar. This works when you have search, but if you have searched for something then the arrow keys just move your cursor in the search box. This behavior was not what I expected when I read the docs.

The currently-selected search result often may have a sub-list. If you hit `enter` to select something (for example, the Ruby `String` class), then it will show a hamburger menu in the search result and focus the main page. There is no way via keyboard alone to click the hamburger menu, and it would be nice if that was possible. In fact, that was what I assumed the `←` and `→` keys would do when I first read the documentation.