“Display as dropdown” option in Categories Widget hides the dropdown

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.]