This would be normal behaviour if there is a large object on spread 4, that extends all the way down.
Thank you for that helpful tip! There was indeed a little object I can't recall how the heck I happened to create and put right down the far edge of the gap (it is also visible in the image I've uploaded).
I have the felling my mighty-mouse is to blame for, while it is not responding as quickly as expected, thus making me instinctively re-click and re-drag ever so often.
Thanks to Peter to, I appreciated that article, which I'm aware of, now!