03 / fixed

background-attachment: fixed, no min-height. The tile is sized to the viewport, so the seam cannot happen at any content height — check short, it is clean. Painted area drops to viewport-scale for free.

Then pull past an end in tall. A fixed background paints only inside the viewport rectangle, so overscroll falls through tobackground-color.

Note the band has changed character rather than gone: on 01 and 02 it was two mismatched tile edges, here it is one flat colour against a gradient. Which invites tuning that colour — and cannot work. The fallback is fixed and the gradient at that edge is mid-animation. Charcoal reads as a light band, near-black as a dark one. Both were tried.