I think the top-line clipping experiment should be pushed forward, or
terminated. For some time now, Gnus line-by-line scrolling has been
broken, and I've seen breakage in other applications. On the other
hand, I see no useful new functionality -- for one, scrolling is still
line oriented.
Andy, if you don't feel like fixing Gnus or investigating why things
break, shouldn't you at least provide a way to turn off the new
feature?