So when you use the comment shortcut C, you get the comment box appearing. However, you will get a weird black box.
Here's a picture of before:
And after reloading, it disappears! This seems weird.
And after looking at the HTML, this seems to be the problem:
<div data-questionid="260187" id="question" class="question keyboard-selected">
After removing the class attribute, the box goes away.