Top line clipping destroys Gnus

Another newspaper-headline-like subject. To see this, view an article that is longer than your screen and that contains an x-face. Then press RET to scroll down by one line (I often read that way.) After the first RET, the scrolling is correct (wow, the top line actually clipped!) However, the second time Gnus is completely confused, and jumps by a whole page. Now, this may very well be a Gnus bug, but even in that case the fact that Gnus got screwed with the change may prove indicative. Andy, could you please describe the exact behavioural change that your top-line-clipping patch induces? I've seen the patch, but I don't remember seeing a description. Not being a redisplay expert, a description would help *a lot*.