Found Chrome 43, FF 38. Also checked with FF39(Windows 8) to make sure that it's not Mac specific.

1. On http://en.wikipedia.beta.wmflabs.org/wiki/Talk:ET1 - click in 'Browse topics' - the list of the topics is displayed.
2. Hover your mouse over the first topic on that list, so it becomes displayed with a grey background.

3 Keep pressing down arrow button on your keyboard to keep moving down the displayed list of topics.

4. When you reach the last displayed topic, the selection jumps up. And the list of topics will display only one added topic at the bottom of the list.

When using the arrow keys to navigate up - it's even more confusing to keep track which topics have been already scrolled up.