Currently, the Jump To Latest pop-up button appears if you scroll back numerous messages - when the scroll bar is about halfway up the page, from what I can tell.
Instead it should appear as soon as there is just ONE message hidden below the fold. That way if new messages come in and the scroll bar is only slightly scrolled up, the user can easily tell that there are new messages.
I've been missing new messages in teams I'm looking at in New RC because of this. The scroll bar will become locked, but it also disappears until you hover over it, so I have no visual indication that there are new messages.