Previous new button doesn't work correctly

Problem/Motivation

When a new comment arrives through polling that is above the current viewport, the "previous" button is supposed to become enabled, and clicking it scrolls you to the previous comment.

Instead, the "Next" button enables. Clicking that does scroll you to the comment, and then if there are additional new comments, takes you to them as expected -- it's just the previous button that fails to work as expected.

Steps to reproduce

Load a page with a dozen or so comments in two different windows, rendered with ACT, with polling enabled.

In one window, scroll down to the bottom.

In the other, add a reply to a comment near the top

Wait for the poll action, and see the "Next" button enable instead of the "previous".

Proposed resolution

Identify/fix the JS that handles this.

Remaining tasks

User interface changes

I am thinking these buttons should hide entirely when there is no new message, and only get shown when there is a new message.

API changes

Data model changes