As a code review author, while responding to comments, I accidentally clicked the "X" button on a comment, hiding it. I wanted to respond to that comment, but could find no way to unhide it.

There should be a way to unhide a comment, possibly an ephemeral "Undo" link that appears after you click the hide button, or some sort of action linked to the line number. Or, clicking the reply button on that comment in the review thread could bring it back. Right now that button seems to do nothing.