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+<!doctype html>
+<meta charset="utf-8">
+<title>28 IntersectionObserver visibility</title>
+<link rel="stylesheet" href="../_style.css">
+<script src="../_lib.js"></script>
+<body>
+<main>
+  <p>Goal: scroll until the hidden <b>TARGET</b> button comes into view, then click it.
+     Page uses an <code>IntersectionObserver</code> to track when TARGET crosses the
+     viewport. Real scrolling produces a sequence of intersection updates with
+     gradually increasing <code>intersectionRatio</code> (0 → 1) across animation
+     frames. Programmatic teleports (<code>scrollIntoView({behavior:"instant"})</code>,
+     direct <code>scrollTop=</code>) produce exactly one observer callback with
+     <code>ratio≈1</code> and no rAF samples in between — fail.</p>
+  <div id="scroller" style="height:240px;overflow:auto;border:1px solid #555;background:#fff;color:#111">
+    <div style="height:1600px;padding:8px">keep scrolling …</div>
+    <button id="target" style="margin:0 auto 1600px;display:block;padding:14px 22px;background:#1f7a1f;color:#fff;border:0;border-radius:6px">TARGET</button>
+  </div>
+</main>
+<script>
+Challenge.init({ id: "intersection-visibility", instructions: "scroll until TARGET visible, then click" });
+
+const scroller = document.getElementById("scroller");
+const target = document.getElementById("target");
+
+const ratioSamples = [];
+const rafSamples = [];
+
+const io = new IntersectionObserver((entries) => {
+  for (const e of entries) ratioSamples.push({ r: e.intersectionRatio, t: performance.now() });
+}, { root: scroller, threshold: Array.from({length: 21}, (_,i) => i/20) });
+io.observe(target);
+
+// Track rAF ticks during which scroll was happening.
+let lastScrollTop = scroller.scrollTop;
+let scrollingFrames = 0;
+function tick() {
+  if (scroller.scrollTop !== lastScrollTop) {
+    scrollingFrames++;
+    rafSamples.push({ st: scroller.scrollTop, t: performance.now() });
+    lastScrollTop = scroller.scrollTop;
+  }
+  requestAnimationFrame(tick);
+}
+requestAnimationFrame(tick);
+
+target.addEventListener("click", (e) => {
+  if (!e.isTrusted) return Challenge.fail("TARGET click isTrusted=false");
+
+  // Need the target to actually be in the viewport at click time.
+  const rect = target.getBoundingClientRect();
+  const sRect = scroller.getBoundingClientRect();
+  const inViewport = rect.top < sRect.bottom && rect.bottom > sRect.top;
+  if (!inViewport) return Challenge.fail("TARGET clicked while still off-screen");
+
+  // Require at least a gradient of intersection ratios from low to high.
+  const lows  = ratioSamples.filter(s => s.r > 0 && s.r < 0.3).length;
+  const mids  = ratioSamples.filter(s => s.r >= 0.3 && s.r < 0.7).length;
+  const highs = ratioSamples.filter(s => s.r >= 0.7).length;
+  Challenge.log("io", { samples: ratioSamples.length, lows, mids, highs, scrollingFrames });
+
+  const bad = [];
+  if (ratioSamples.length < 4) bad.push(`only ${ratioSamples.length} IntersectionObserver samples (teleport)`);
+  if (lows === 0)  bad.push("no low-ratio IO samples — target appeared instantly");
+  if (mids === 0)  bad.push("no mid-ratio IO samples — no smooth approach");
+  if (scrollingFrames < 5) bad.push(`only ${scrollingFrames} rAF frames saw scroll motion`);
+
+  if (bad.length) Challenge.fail(...bad);
+  else Challenge.pass(`IO gradient seen: lows=${lows} mids=${mids} highs=${highs}, rafFrames=${scrollingFrames}`);
+});
+</script>
+</body>