In case that title isn't clear, here's what I did:

a.) in a PR, add a comment on a line. click the submit button
b.) while the spinner is going and the comment hasn't fully submitted, click another line to add a comment
c.) the mid-submission comment field will jump down to the new line; it will still submit correctly, though, on the line where it was originally submitted (as refreshing the page confirms). Editing the comment while it's in the incorrect location still edits the comment at the other location; for all purposes except the display before refreshing the page, the comment is at the original location.