Scroll-Linked 3D Scene

:root {
  --bg: #0c0d10;
  --surface: #15161b;
  --surface-2: #1d1f27;
  --ink: #e9eaf0;
  --muted: #9a9cab;
  --accent: #8b5cf6;
  --accent-2: #22d3ee;
  --ok: #34d399;
  --warn: #fbbf24;
  --danger: #f87171;
  --line: rgba(255, 255, 255, 0.10);
  --line-2: rgba(255, 255, 255, 0.18);
  --r-sm: 8px;
  --r-md: 12px;
  --r-lg: 16px;
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: "Inter", system-ui, -apple-system, sans-serif;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  overflow-x: hidden;
}

code {
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.9em;
  background: var(--surface-2);
  border: 1px solid var(--line);
  padding: 0.05em 0.4em;
  border-radius: 6px;
  color: var(--accent-2);
}

/* ---------- Top bar ---------- */
.topbar {
  position: sticky;
  top: 0;
  z-index: 40;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  padding: 14px 22px;
  background: rgba(12, 13, 16, 0.72);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--line);
}

.brand {
  display: flex;
  align-items: center;
  gap: 10px;
  font-size: 15px;
  font-weight: 600;
  letter-spacing: -0.01em;
}

.brand strong {
  color: var(--accent);
}

.brand .dot {
  width: 10px;
  height: 10px;
  border-radius: 50%;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  box-shadow: 0 0 14px rgba(139, 92, 246, 0.7);
}

.hint {
  margin: 0;
  font-size: 12.5px;
  color: var(--muted);
}

/* ---------- Sections ---------- */
main {
  max-width: 100%;
}

.lede,
.outro {
  max-width: 720px;
  margin: 0 auto;
  padding: 96px 24px;
  text-align: center;
}

.lede {
  padding-top: 120px;
}

.lede h1 {
  font-size: clamp(28px, 5vw, 46px);
  line-height: 1.1;
  letter-spacing: -0.02em;
  margin: 0 0 20px;
  background: linear-gradient(180deg, var(--ink), var(--muted));
  -webkit-background-clip: text;
  background-clip: text;
  color: transparent;
}

.lede p,
.outro p {
  font-size: 16px;
  color: var(--muted);
  margin: 0 auto;
  max-width: 560px;
}

.outro h2 {
  font-size: clamp(22px, 4vw, 30px);
  letter-spacing: -0.01em;
  margin: 0 0 16px;
}

.scroll-cue {
  margin-top: 40px;
  display: inline-flex;
  flex-direction: column;
  align-items: center;
  gap: 6px;
  color: var(--accent-2);
  font-size: 12px;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.14em;
}

.scroll-cue svg {
  animation: bob 1.8s ease-in-out infinite;
}

@keyframes bob {
  0%, 100% { transform: translateY(0); opacity: 0.6; }
  50% { transform: translateY(6px); opacity: 1; }
}

/* ---------- Scroll track + sticky stage ---------- */
.track {
  position: relative;
  height: 420vh; /* tall track = scroll runway */
}

.sticky {
  position: sticky;
  top: 0;
  height: 100vh;
  display: grid;
  grid-template-rows: auto 1fr auto;
  align-items: center;
  overflow: hidden;
  border-top: 1px solid var(--line);
  border-bottom: 1px solid var(--line);
  background:
    radial-gradient(120% 80% at 50% -10%, rgba(139, 92, 246, 0.16), transparent 60%),
    radial-gradient(90% 70% at 50% 120%, rgba(34, 211, 238, 0.12), transparent 60%),
    var(--bg);
}

/* subtle dotted backdrop */
.sticky::before {
  content: "";
  position: absolute;
  inset: 0;
  background-image: radial-gradient(var(--line) 1px, transparent 1px);
  background-size: 26px 26px;
  mask-image: radial-gradient(70% 60% at 50% 45%, #000 40%, transparent 80%);
  opacity: 0.5;
  pointer-events: none;
}

/* ---------- Controls ---------- */
.controls {
  position: relative;
  z-index: 5;
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 10px;
  padding: 22px 16px 4px;
}

.ctrl {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  font-family: inherit;
  font-size: 13px;
  font-weight: 500;
  color: var(--ink);
  background: var(--surface);
  border: 1px solid var(--line-2);
  border-radius: 999px;
  padding: 8px 14px;
  cursor: pointer;
  transition: border-color 0.16s ease, background 0.16s ease, transform 0.12s ease;
}

.ctrl:hover {
  border-color: var(--accent);
  background: var(--surface-2);
}

.ctrl:active {
  transform: translateY(1px);
}

.ctrl:focus-visible {
  outline: 2px solid var(--accent-2);
  outline-offset: 2px;
}

.ctrl-key {
  color: var(--muted);
}

.ctrl-val {
  color: var(--accent-2);
  font-weight: 600;
}

.ctrl-accent {
  background: linear-gradient(135deg, rgba(139, 92, 246, 0.22), rgba(34, 211, 238, 0.16));
  border-color: var(--accent);
}

/* ---------- 3D Stage ---------- */
.stage {
  position: relative;
  z-index: 4;
  display: grid;
  place-items: center;
  perspective: 1200px;
  perspective-origin: 50% 44%;
  padding: 8px;
}

.scene {
  position: relative;
  width: min(280px, 62vw);
  height: min(280px, 62vw);
  transform-style: preserve-3d;
  will-change: transform;
}

.layer {
  position: absolute;
  inset: 0;
  transform-style: preserve-3d;
  will-change: transform;
}

.panel {
  position: absolute;
  inset: 0;
  border-radius: var(--r-lg);
  border: 1px solid var(--line-2);
  background: var(--surface);
  box-shadow:
    0 20px 50px -20px rgba(0, 0, 0, 0.8),
    inset 0 1px 0 rgba(255, 255, 255, 0.06);
  overflow: hidden;
  backface-visibility: hidden;
}

.panel .tag {
  position: absolute;
  left: 12px;
  bottom: 10px;
  font-size: 11px;
  font-weight: 600;
  letter-spacing: 0.02em;
  color: var(--ink);
  background: rgba(12, 13, 16, 0.6);
  border: 1px solid var(--line);
  padding: 3px 8px;
  border-radius: 999px;
  backdrop-filter: blur(4px);
}

/* Substrate */
.panel-base {
  background:
    linear-gradient(135deg, #202128, #14151a);
}
.grid-face {
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(var(--line) 1px, transparent 1px),
    linear-gradient(90deg, var(--line) 1px, transparent 1px);
  background-size: 22px 22px;
  opacity: 0.7;
}

/* Logic board */
.panel-board {
  background: linear-gradient(135deg, #0f2b24, #0c1a17);
}
.traces {
  position: absolute;
  inset: 0;
  background:
    repeating-linear-gradient(90deg, transparent 0 18px, rgba(52, 211, 153, 0.28) 18px 19px),
    repeating-linear-gradient(0deg, transparent 0 30px, rgba(52, 211, 153, 0.18) 30px 31px);
  mask-image: radial-gradient(80% 80% at 50% 50%, #000 30%, transparent 90%);
}

/* Cells */
.panel-cells {
  background: linear-gradient(135deg, #2a1e0d, #1a1408);
}
.cells {
  position: absolute;
  inset: 14px;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 8px;
}
.cells::before,
.cells::after { content: ""; }
.cells {
  background-image:
    linear-gradient(rgba(251, 191, 36, 0.3), rgba(251, 191, 36, 0.05));
  border-radius: var(--r-md);
  box-shadow: inset 0 0 0 1px rgba(251, 191, 36, 0.25);
}

/* Display */
.panel-screen {
  background: linear-gradient(160deg, #171826, #101120);
}
.ui-mock {
  position: absolute;
  inset: 16px;
  display: flex;
  flex-direction: column;
  gap: 10px;
}
.ui-bar {
  height: 10px;
  width: 55%;
  border-radius: 6px;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
}
.ui-row {
  height: 8px;
  border-radius: 6px;
  background: var(--line-2);
}
.ui-row.short { width: 60%; }
.ui-chip {
  margin-top: auto;
  height: 26px;
  width: 92px;
  border-radius: 8px;
  background: rgba(139, 92, 246, 0.25);
  border: 1px solid var(--accent);
}

/* Cover glass */
.panel-glass {
  background: linear-gradient(135deg, rgba(255, 255, 255, 0.10), rgba(255, 255, 255, 0.02));
  border-color: rgba(255, 255, 255, 0.28);
  backdrop-filter: blur(2px);
}
.glass-glint {
  position: absolute;
  inset: 0;
  background: linear-gradient(115deg, transparent 30%, rgba(255, 255, 255, 0.22) 46%, transparent 60%);
}

/* ---------- Captions ---------- */
.captions {
  position: relative;
  z-index: 5;
  height: 44px;
  display: grid;
  place-items: center;
  margin-bottom: 28px;
}

.cap {
  grid-area: 1 / 1;
  margin: 0;
  font-size: 13.5px;
  color: var(--muted);
  font-weight: 500;
  text-align: center;
  padding: 0 16px;
  opacity: 0;
  transform: translateY(6px);
  transition: opacity 0.35s ease, transform 0.35s ease;
}

.cap.on {
  opacity: 1;
  transform: translateY(0);
}

.cap.on strong,
.cap b { color: var(--ink); }

/* ---------- Progress rail ---------- */
.rail {
  position: fixed;
  right: 18px;
  top: 50%;
  transform: translateY(-50%);
  z-index: 30;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 10px;
  opacity: 0;
  transition: opacity 0.3s ease;
}

.rail.show { opacity: 1; }

.rail-track {
  position: relative;
  width: 5px;
  height: 190px;
  border-radius: 999px;
  background: var(--surface-2);
  border: 1px solid var(--line);
  overflow: hidden;
}

.rail-fill {
  position: absolute;
  left: 0;
  bottom: 0;
  width: 100%;
  height: 0%;
  background: linear-gradient(180deg, var(--accent-2), var(--accent));
  border-radius: 999px;
  transition: height 0.08s linear;
}

.rail-meta {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 2px;
}

.rail-pct {
  font-family: "JetBrains Mono", monospace;
  font-size: 12px;
  font-weight: 500;
  color: var(--ink);
}

.rail-stage {
  font-size: 10px;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--muted);
  max-width: 60px;
  text-align: center;
}

/* ---------- Reduced-motion fallback ---------- */
.rm-fallback {
  position: absolute;
  inset: 0;
  z-index: 6;
  display: grid;
  place-items: center;
  padding: 24px;
}

.rm-card {
  grid-area: 1 / 1;
  width: min(300px, 74vw);
  height: 200px;
  border-radius: var(--r-lg);
  background: var(--surface);
  border: 1px solid var(--line-2);
  display: grid;
  place-items: center;
  font-size: 17px;
  font-weight: 600;
  color: var(--ink);
  box-shadow: 0 24px 60px -24px rgba(0, 0, 0, 0.8);
  opacity: 0;
  transition: opacity 0.4s ease;
}

.rm-card.on { opacity: 1; }

.rm-note {
  position: absolute;
  bottom: 28px;
  left: 0;
  right: 0;
  text-align: center;
  font-size: 12.5px;
  color: var(--muted);
  margin: 0;
}

.sr-progress {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
}

/* ---------- Responsive ---------- */
@media (max-width: 520px) {
  .topbar { flex-direction: column; align-items: flex-start; gap: 4px; }
  .hint { font-size: 11.5px; }
  .lede, .outro { padding: 72px 20px; }
  .track { height: 380vh; }
  .rail { right: 10px; }
  .rail-track { height: 140px; }
  .controls { gap: 8px; }
  .ctrl { font-size: 12px; padding: 7px 12px; }
  .scene { width: 66vw; height: 66vw; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  .scroll-cue svg { animation: none; }
}

(function () {
  "use strict";

  var track = document.getElementById("track");
  var scene = document.getElementById("scene");
  var layers = Array.prototype.slice.call(document.querySelectorAll(".layer"));
  var caps = Array.prototype.slice.call(document.querySelectorAll(".cap"));
  var railFill = document.getElementById("railFill");
  var railPct = document.getElementById("railPct");
  var railStage = document.getElementById("railStage");
  var rail = document.querySelector(".rail");
  var srProgress = document.getElementById("srProgress");
  var rmFallback = document.getElementById("rmFallback");
  var rmCards = Array.prototype.slice.call(document.querySelectorAll(".rm-card"));

  var modeBtn = document.getElementById("modeBtn");
  var modeVal = document.getElementById("modeVal");
  var driftBtn = document.getElementById("driftBtn");
  var driftVal = document.getElementById("driftVal");
  var assembleBtn = document.getElementById("assembleBtn");

  var reduceQuery = window.matchMedia("(prefers-reduced-motion: reduce)");

  /* ---- State ---- */
  var mode = "explode"; // "explode" | "rotate"
  var driftOn = true;
  var reduce = reduceQuery.matches;

  var rawP = 0; // scroll-derived progress 0..1
  var dispP = 0; // eased/animated progress actually rendered
  var forced = null; // when snap-to-assembled is active, target progress
  var lastStage = -1;
  var ticking = false;
  var driftPhase = 0;

  /* ---- Per-layer explode config (Z depth + parallax offset) ---- */
  var layerCfg = [
    { z: -160, ox: -0.9, oy: 0.7 },
    { z: -80, ox: -0.5, oy: 0.35 },
    { z: 0, ox: 0, oy: 0 },
    { z: 90, ox: 0.55, oy: -0.4 },
    { z: 190, ox: 1.0, oy: -0.8 }
  ];

  var stageLabels = ["idle", "tilt", "explode", "inspect"];

  function clamp(v, a, b) {
    return v < a ? a : v > b ? b : v;
  }

  function lerp(a, b, t) {
    return a + (b - a) * t;
  }

  /* Compute scroll progress of the sticky stage through its track. */
  function computeProgress() {
    var rect = track.getBoundingClientRect();
    var vh = window.innerHeight || document.documentElement.clientHeight;
    // total scrollable distance while the stage is pinned
    var scrollable = rect.height - vh;
    if (scrollable <= 0) return 0;
    var travelled = clamp(-rect.top, 0, scrollable);
    return travelled / scrollable;
  }

  /* Ease progress a touch for buttery scrub without lag. */
  function ease(t) {
    // smoothstep
    return t * t * (3 - 2 * t);
  }

  function render() {
    ticking = false;

    if (reduce) {
      renderReduced();
      return;
    }

    // choose target: forced snap or live scroll
    var target = forced != null ? forced : rawP;
    // approach target for smooth motion
    dispP += (target - dispP) * 0.18;
    if (Math.abs(target - dispP) < 0.0008) dispP = target;

    var p = clamp(dispP, 0, 1);
    var e = ease(p);

    /* Scene rotation: builds up over first ~40% then holds/rotates. */
    var rotX = lerp(2, 18, ease(clamp(p / 0.45, 0, 1)));
    var rotY = mode === "rotate"
      ? lerp(-30, 30, e)
      : lerp(-18, 22, e);

    var drift = 0;
    if (driftOn) {
      driftPhase += 0.008;
      drift = Math.sin(driftPhase) * 4;
    }

    scene.style.transform =
      "rotateX(" + rotX.toFixed(2) + "deg) rotateY(" + (rotY + drift).toFixed(2) + "deg)";

    /* Explode factor: 0 until 25% progress, ramps to 1 by 95%. */
    var explode = mode === "explode"
      ? ease(clamp((p - 0.22) / 0.7, 0, 1))
      : ease(clamp((p - 0.4) / 0.5, 0, 1)) * 0.35;

    for (var i = 0; i < layers.length; i++) {
      var cfg = layerCfg[i];
      var z = cfg.z * explode;
      var tx = cfg.ox * 46 * explode;
      var ty = cfg.oy * 40 * explode;
      layers[i].style.transform =
        "translate3d(" + tx.toFixed(1) + "px," + ty.toFixed(1) + "px," + z.toFixed(1) + "px)";
    }

    updateHud(p);

    // keep animating while drifting or approaching target
    if (driftOn || forced != null || Math.abs((forced != null ? forced : rawP) - dispP) > 0.0008) {
      requestFrame();
    }
  }

  function renderReduced() {
    var p = clamp(rawP, 0, 1);
    var idx = Math.min(rmCards.length - 1, Math.floor(p * rmCards.length));
    for (var i = 0; i < rmCards.length; i++) {
      rmCards[i].classList.toggle("on", i === idx);
    }
    updateHud(p);
  }

  function updateHud(p) {
    var pct = Math.round(p * 100);
    railFill.style.height = pct + "%";
    railPct.textContent = pct + "%";

    var stage = clamp(Math.floor(p * 4), 0, 3);
    railStage.textContent = stageLabels[stage];

    if (stage !== lastStage) {
      lastStage = stage;
      for (var i = 0; i < caps.length; i++) {
        caps[i].classList.toggle("on", i === stage);
      }
      srProgress.textContent =
        "Assembly stage " + (stage + 1) + " of 4, " + pct + " percent.";
    }
  }

  var frameReq = null;
  function requestFrame() {
    if (ticking) return;
    ticking = true;
    frameReq = requestAnimationFrame(render);
  }

  /* ---- Scroll: only store value, render in rAF ---- */
  function onScroll() {
    rawP = computeProgress();
    if (forced != null) {
      // any manual scroll cancels the snap
      forced = null;
      assembleBtn.classList.remove("ctrl-accent-active");
    }
    requestFrame();
  }

  /* ---- Rail visibility tied to the track ---- */
  var io = null;
  if ("IntersectionObserver" in window) {
    io = new IntersectionObserver(
      function (entries) {
        rail.classList.toggle("show", entries[0].isIntersecting);
      },
      { threshold: 0.02 }
    );
    io.observe(track);
  } else {
    rail.classList.add("show");
  }

  /* ---- Controls ---- */
  modeBtn.addEventListener("click", function () {
    mode = mode === "explode" ? "rotate" : "explode";
    modeVal.textContent = mode === "explode" ? "Explode" : "Rotate";
    modeBtn.dataset.mode = mode;
    modeBtn.setAttribute("aria-pressed", String(mode === "explode"));
    requestFrame();
  });

  driftBtn.addEventListener("click", function () {
    driftOn = !driftOn;
    driftVal.textContent = driftOn ? "On" : "Off";
    driftBtn.setAttribute("aria-pressed", String(driftOn));
    requestFrame();
  });

  assembleBtn.addEventListener("click", function () {
    // toggle between fully assembled (0) and fully exploded (1)
    forced = forced === 0 ? 1 : 0;
    assembleBtn.textContent = forced === 0 ? "Snap to exploded" : "Snap to assembled";
    requestFrame();
  });

  /* ---- Reduced-motion setup + live toggle ---- */
  function applyMotionPref() {
    reduce = reduceQuery.matches;
    if (reduce) {
      rmFallback.hidden = false;
      scene.parentElement.style.display = "none";
      caps.forEach(function (c) { c.style.display = "none"; });
    } else {
      rmFallback.hidden = true;
      scene.parentElement.style.display = "";
      caps.forEach(function (c) { c.style.display = ""; });
    }
    lastStage = -1;
    requestFrame();
  }

  if (reduceQuery.addEventListener) {
    reduceQuery.addEventListener("change", applyMotionPref);
  } else if (reduceQuery.addListener) {
    reduceQuery.addListener(applyMotionPref);
  }

  /* ---- Init ---- */
  window.addEventListener("scroll", onScroll, { passive: true });
  window.addEventListener("resize", onScroll);
  applyMotionPref();
  onScroll();
})();

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title>Scroll-Linked 3D Scene</title>
  <link rel="preconnect" href="https://fonts.googleapis.com" />
  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
  <link
    href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500&display=swap"
    rel="stylesheet"
  />
  <link rel="stylesheet" href="style.css" />
</head>
<body>
  <header class="topbar">
    <div class="brand">
      <span class="dot" aria-hidden="true"></span>
      <span>Scroll-Linked <strong>3D</strong> Scene</span>
    </div>
    <p class="hint">Scroll the page to scrub the assembly &middot; sticky viewport below</p>
  </header>

  <main>
    <!-- Intro -->
    <section class="lede">
      <h1>Scrub a 3D exploded view with your scroll</h1>
      <p>
        The viewport stays pinned while a tall track drives a normalized progress
        value <code>0 &rarr; 1</code>. That single number rotates the cluster, explodes
        the layers along&nbsp;Z, and stages the captions — pure CSS 3D transforms,
        no WebGL.
      </p>
      <div class="scroll-cue" aria-hidden="true">
        <span>Scroll</span>
        <svg viewBox="0 0 24 24" width="18" height="18"><path d="M12 5v14M6 13l6 6 6-6" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/></svg>
      </div>
    </section>

    <!-- The tall scroll track. The sticky stage lives inside. -->
    <section class="track" id="track" aria-label="Scroll-linked 3D assembly">
      <div class="sticky">
        <!-- Controls -->
        <div class="controls" role="group" aria-label="Scene controls">
          <button type="button" id="modeBtn" class="ctrl" aria-pressed="true" data-mode="explode">
            <span class="ctrl-key">Mode</span><span class="ctrl-val" id="modeVal">Explode</span>
          </button>
          <button type="button" id="driftBtn" class="ctrl" aria-pressed="true">
            <span class="ctrl-key">Idle drift</span><span class="ctrl-val" id="driftVal">On</span>
          </button>
          <button type="button" id="assembleBtn" class="ctrl ctrl-accent">
            Snap to assembled
          </button>
        </div>

        <!-- 3D stage -->
        <div class="stage" id="stage" aria-hidden="false">
          <div class="scene" id="scene">
            <div class="layer" data-layer="0">
              <div class="panel panel-base">
                <div class="grid-face"></div>
                <span class="tag">Substrate</span>
              </div>
            </div>
            <div class="layer" data-layer="1">
              <div class="panel panel-board">
                <div class="traces"></div>
                <span class="tag">Logic board</span>
              </div>
            </div>
            <div class="layer" data-layer="2">
              <div class="panel panel-cells">
                <div class="cells"></div>
                <span class="tag">Cells</span>
              </div>
            </div>
            <div class="layer" data-layer="3">
              <div class="panel panel-screen">
                <div class="ui-mock">
                  <div class="ui-bar"></div>
                  <div class="ui-row"></div>
                  <div class="ui-row short"></div>
                  <div class="ui-chip"></div>
                </div>
                <span class="tag">Display</span>
              </div>
            </div>
            <div class="layer" data-layer="4">
              <div class="panel panel-glass">
                <div class="glass-glint"></div>
                <span class="tag">Cover glass</span>
              </div>
            </div>
          </div>
        </div>

        <!-- Stage caption, staged by progress -->
        <div class="captions" aria-live="polite">
          <p class="cap" data-cap="0">01 &middot; Layers stacked flat, awaiting assembly.</p>
          <p class="cap" data-cap="1">02 &middot; Cluster tilts into perspective view.</p>
          <p class="cap" data-cap="2">03 &middot; Panels explode apart along the Z axis.</p>
          <p class="cap" data-cap="3">04 &middot; Fully separated — inspect every layer.</p>
        </div>

        <!-- Reduced-motion fallback -->
        <div class="rm-fallback" id="rmFallback" hidden>
          <div class="rm-card" data-rm="0"><span>Substrate</span></div>
          <div class="rm-card" data-rm="1"><span>Logic board</span></div>
          <div class="rm-card" data-rm="2"><span>Cells</span></div>
          <div class="rm-card" data-rm="3"><span>Display + glass</span></div>
          <p class="rm-note">Reduced-motion: layers crossfade instead of moving in 3D.</p>
        </div>
      </div>
    </section>

    <!-- Outro -->
    <section class="outro">
      <h2>How it works</h2>
      <p>
        A clamped ratio of how far the sticky stage has travelled through its track
        becomes <code>p</code>. Transforms are interpolated from <code>p</code> and
        applied once per frame inside <code>requestAnimationFrame</code> — the scroll
        handler only stores the latest position, so there is no per-event layout thrash.
      </p>
    </section>
  </main>

  <!-- Fixed progress rail -->
  <aside class="rail" aria-hidden="true">
    <div class="rail-track"><span class="rail-fill" id="railFill"></span></div>
    <div class="rail-meta">
      <span class="rail-pct" id="railPct">0%</span>
      <span class="rail-stage" id="railStage">idle</span>
    </div>
  </aside>

  <div class="sr-progress" role="status" aria-live="off" id="srProgress"></div>

  <script src="script.js"></script>
</body>
</html>