I've never used this option, and I was testing it on my theme but when I clicked the "Display as dropdown" checkbox and went to see what the widget looks like - it was hidden. The HTML code looked like this:
<select name="cat" id="cat" class="postform" style="z-index: 10; opacity: 0;"> ... </select>
Why is it automatically appending opacity: 0 ?
[No bumping, thank you.]