1.open a topic with multiple posts
2.press the key J down it will start moving to next item repeatedly.Until there are no more items it will stop at the bottom of the page
at this point you should see that the scrolling and scroll bar is unstable, scroll bar jumps and change sizes.
You can repeat the process from the last item back to the top first item
3.press the key K down it will start moving to previous item, until there are no more items it will stop at the top
Tested on: Chrome, Firefox / Win