Where the gradient stops

background-size: 1000% sets width only, so the height resolves to 100% of the box: one tile, its top and bottom edges at opposite ends of a-45deg ramp, repeating. Everywhere the canvas paints outside that tile, those two edges meet. There are two such places — below a page shorter than the viewport, and past either end of a scroll — so one fault presents as two unrelated-looking bugs and invites two separate wrong fixes. 02 is the wrong fix, kept because it was nearly shipped.

Use dark mode and the probe toggle. The shipped gradient is 13 stops of which 8 are flat near-black, drawn at 1000% and animating, so whether a fault lands on a visible colour depends on the window size and which second of the cycle you look — the probe swaps in one hard tile so the geometry can be read instead of waited for. Nothing here shows up in a screenshot of a long page: the seam needs the page shorter than the viewport, the band needs you to physically pull past an edge.