The following occurs fairly often: I start writing a comment in (the new) Basecamp and then decide not to submit it. How do I exit from the comment text box?
Just click refresh in your browser :). This will repaint the page with the comment box collapsed. Or feel free to navigate away from the page.
There's no harm or impact to having the comment box displayed. Text that you have typed will be auto-saved as draft, but not officially saved/committed.