I think the scrolling thing may be more a Windows problem than a K-M one; it happens for me in other programs too. Depends on how cranky your Winders is.
I have seen the White Line, but not very often. It goes away on scrolling.
Just re-installed K-M an hour ago, after a couple of days of playing Drive Pong (not a fun game) and making fresh Windows. I'll be watching it closely. ):-]