Goal: scroll until the hidden TARGET button comes into view, then click it. Page uses an IntersectionObserver to track when TARGET crosses the viewport. Real scrolling produces a sequence of intersection updates with gradually increasing intersectionRatio (0 → 1) across animation frames. Programmatic teleports (scrollIntoView({behavior:"instant"}), direct scrollTop=) produce exactly one observer callback with ratio≈1 and no rAF samples in between — fail.