There appears to be an issue with how the LookupElement clears its result menu box, this issue was found as part of the AdvancedSearch extension. Whenever users use backspace to clear all search results or click on a search result, the result menu box clears its content but it does not hide itself. This leaves behind an empty box below/above the input field.
(Note: This ticket description was changed after another related issue was solved externally, this description is now up-to-date and describes a leftover problem. The initial ticket mentioned that this was limited to Timeless and Chromium but it affects all skins / browsers)