Keyframe Scrub Stack

<!--
  Keyframe Scrub Stack - advanced HyperFrames primitive.

  This is intentionally seekable: drive it from a paused GSAP timeline.
  MotionPath examples require GSAP MotionPathPlugin. Three.js examples expose
  a render(progress) adapter instead of a free-running requestAnimationFrame loop.
-->

<div class="hf-advanced-keyframe-scrub-stack">
  <div class="stack">
    <div class="card" style="--i: 0"></div>
    <div class="card" style="--i: 1"></div>
    <div class="card" style="--i: 2"></div>
  </div>
</div>

<style>
  .hf-advanced-keyframe-scrub-stack {
    --hf-accent: var(--accent, rgba(17, 24, 39, 0.72));
    --hf-l1: rgba(17, 24, 39, 0.72);
    --hf-l3: rgba(17, 24, 39, 0.18);
    --hf-l4: rgba(17, 24, 39, 0.08);
    --hf-hair: rgba(17, 24, 39, 0.14);
    color: var(--hf-l1);
    font-family: Inter, system-ui, sans-serif;
  }
  .hf-advanced-keyframe-scrub-stack {
    width: 720px;
    min-height: 420px;
    border-radius: 30px;
    padding: 36px;
    background: #fff;
    box-shadow: 0 28px 80px rgba(15, 23, 42, 0.16);
    display: grid;
    place-items: center;
  }
  .hf-advanced-keyframe-scrub-stack .grid {
    display: grid;
    grid-template-columns: repeat(8, 46px);
    gap: 10px;
  }
  .hf-advanced-keyframe-scrub-stack .cell {
    width: 46px;
    height: 46px;
    border-radius: 14px;
    background: var(--hf-l1);
    transform: translateY(var(--hf-cell-y, 0px)) scale(var(--hf-cell-scale, 1));
    opacity: var(--hf-cell-opacity, 1);
  }
  .hf-advanced-keyframe-scrub-stack svg {
    width: 520px;
    height: 220px;
    overflow: visible;
  }
  .hf-advanced-keyframe-scrub-stack path {
    fill: none;
    stroke: var(--hf-accent);
    stroke-width: 14;
    stroke-linecap: round;
    stroke-dasharray: 900;
    stroke-dashoffset: var(--hf-dash, 0);
  }
  .hf-advanced-keyframe-scrub-stack .stack {
    position: relative;
    width: 520px;
    height: 260px;
  }
  .hf-advanced-keyframe-scrub-stack .card {
    position: absolute;
    inset: 0;
    border-radius: 26px;
    background: linear-gradient(var(--hf-l4), var(--hf-l4)), #ffffff;
    border: 1px solid var(--hf-hair);
    box-shadow: 0 20px 54px rgba(15, 23, 42, 0.14);
    transform: translateY(var(--hf-card-y, 0px)) scale(var(--hf-card-scale, 1));
    opacity: var(--hf-card-opacity, 1);
  }
</style>

<!--
  Timeline integration:
  const startTime = 0;
  tl.fromTo('.hf-advanced-keyframe-scrub-stack .card', { '--hf-card-y': '42px', '--hf-card-scale': 0.88, '--hf-card-opacity': 0 }, { '--hf-card-y': (i) => (i * 26) + 'px', '--hf-card-scale': (i) => 1 - i * 0.04, '--hf-card-opacity': 1, duration: 0.48, stagger: 0.12, ease: 'back.out(1.6)' }, startTime);
-->