The list of suggestions created by OO.ui.mixin.LookupElement has a strange padding at the bottom when there is just one suggestion. This padding isn't there for more suggestions.
To reproduce, you can go to https://doc.wikimedia.org/oojs-ui/master/demos/?page=widgets&theme=wikimediaui&direction=ltr&platform=desktop#LookupElement-try-inputting-an-integer and change the code in the following way:
widget.getLookupRequest = function () { var value = this.getValue(), deferred = $.Deferred(), delay = 500 + Math.floor( Math.random() * 500 ); this.getValidity().then( function () { setTimeout( function () { deferred.resolve( [ value * 2 ] ); }, delay ); }, function () { deferred.resolve( [] ); } ); return deferred.promise( { abort: function () {} } ); }This results in something like this:
Note the small area below the highlighted selection.