This made me realize I instinctively move my mouse to the very left side of the screen when I scroll. I think this is why. I don't actually mind it that much, but it would be better if the two feeds were at a fixed length so we can just scroll the full page, no matter where the cursor is.
I mean, the scroll only moves down three more threads. Can it not be locked at the full length for 20 threads? I suppose that makes it look asymmetrical due to variations in title sizes, etc. I might prefer that way anyway, even if there is empty space at the bottom of one of the boxes, but I'm not the best person to comment on design choices.
It probably does look better the way it is now, so I don't have a strong opinion on it.