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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>
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