I thought it had to move the chroma without interpolation.
Oh, I see. You moved the chroma back after making the video mod2 again! Seems so obvious now. Thanks.
edit: it is somewhat embarrassing how well I feel I understand YV12, point resize, etc. and how bad I am at seeing the (in retrospect, at least) obvious consequences.
You are also right, I was only thinking about the chroma when looking at supersampled addboarders (but I didn't think I was).