Panel Reveal

<!--
  Panel Reveal - transitions.dev-inspired primitive for HyperFrames.

  Paste the markup, CSS and script into a composition. Use the timeline
  integration note at the bottom as the starting point for deterministic GSAP
  animation.

  Variables. The script reads each one, falls back to the declared default on
  anything missing or unrecognised, and writes the result as a custom property
  the CSS below consumes. The timeline recipe at the end is untouched by them:
  it keeps driving the single open value from 0 to 1.

    - title (string, default "Motion tokens"): the header line above the fold.
    - width (narrow | standard | wide, default standard): how wide the panel
      sits, 280, 360 or 480 px.
    - tone (paper | ink, default paper): the surface family. paper is the
      white card on a light frame, ink is the same card inverted for dark
      frames, border and body copy included.
    - reveal (fade | solid, default fade): whether the body fades in with the
      height. fade is the shipped motion, solid keeps the body at full opacity
      so only the height opens. Both look the same once the panel is open.

  On every default the result is identical to the original: a 360px white card
  whose body fades in as it grows.
-->

<section
  class="hf-transition-panel-reveal"
  data-composition-variables='[
    { "id": "title", "type": "string", "role": "content", "label": "Title", "description": "The header line that sits above the folded body.", "default": "Motion tokens" },
    { "id": "width", "type": "enum", "role": "layout", "label": "Width", "description": "How wide the panel sits, 280, 360 or 480 px.", "default": "standard", "options": [{ "value": "narrow", "label": "Narrow" }, { "value": "standard", "label": "Standard" }, { "value": "wide", "label": "Wide" }] },
    { "id": "tone", "type": "enum", "role": "style", "label": "Tone", "description": "Surface family: paper is the white card for light frames, ink inverts it for dark frames.", "default": "paper", "options": [{ "value": "paper", "label": "Paper" }, { "value": "ink", "label": "Ink" }] },
    { "id": "reveal", "type": "enum", "role": "motion", "label": "Reveal", "description": "Whether the body fades in with the height. fade is the shipped motion, solid opens the height only.", "default": "fade", "options": [{ "value": "fade", "label": "Fade" }, { "value": "solid", "label": "Solid" }] }
  ]'
>
  <header>Motion tokens</header>
  <div>
    <p>Duration 420ms</p>
    <p>Easing power3.out</p>
  </div>
</section>

<style>
  .hf-transition-panel-reveal {
    --hf-panel-open: 1;
    width: var(--hf-panel-width, 360px);
    overflow: hidden;
    border: 1px solid var(--hf-panel-border, #e4e4e7);
    border-radius: 14px;
    background: var(--hf-panel-surface, #fff);
    color: var(--hf-panel-text, #18181b);
    font-family: Inter, system-ui, sans-serif;
    box-shadow: 0 20px 56px rgba(24, 24, 27, 0.1);
  }
  .hf-transition-panel-reveal header {
    padding: 15px 17px;
    font-weight: 900;
  }
  .hf-transition-panel-reveal div {
    max-height: calc(var(--hf-panel-open) * 120px);
    opacity: calc(1 - var(--hf-panel-fade, 1) * (1 - var(--hf-panel-open)));
    padding: 0 17px calc(var(--hf-panel-open) * 17px);
    overflow: hidden;
  }
  .hf-transition-panel-reveal p {
    margin: 0 0 8px;
    color: var(--hf-panel-muted, #71717a);
    font-size: 14px;
  }
</style>

<script>
  (function () {
    "use strict";

    var vars =
      window.__hyperframes && window.__hyperframes.getVariables
        ? window.__hyperframes.getVariables()
        : {};

    // Unrecognised overrides return to their declared defaults.
    var widths = { narrow: "280px", standard: "360px", wide: "480px" };
    var tones = {
      paper: {
        surface: "#fff",
        border: "#e4e4e7",
        text: "#18181b",
        muted: "#71717a",
      },
      ink: {
        surface: "#18181b",
        border: "#3f3f46",
        text: "#fafafa",
        muted: "#a1a1aa",
      },
    };
    var reveals = { fade: 1, solid: 0 };

    function pick(table, value, fallback) {
      return Object.prototype.hasOwnProperty.call(table, value) ? value : fallback;
    }

    var width = widths[pick(widths, vars.width, "standard")];
    var tone = tones[pick(tones, vars.tone, "paper")];
    var fade = reveals[pick(reveals, vars.reveal, "fade")];

    var title = typeof vars.title === "string" ? vars.title.trim() : "";
    if (!title) {
      title = "Motion tokens";
    }

    var roots = document.querySelectorAll(".hf-transition-panel-reveal");
    for (var i = 0; i < roots.length; i += 1) {
      var root = roots[i];
      root.style.setProperty("--hf-panel-width", width);
      root.style.setProperty("--hf-panel-surface", tone.surface);
      root.style.setProperty("--hf-panel-border", tone.border);
      root.style.setProperty("--hf-panel-text", tone.text);
      root.style.setProperty("--hf-panel-muted", tone.muted);
      root.style.setProperty("--hf-panel-fade", String(fade));

      var header = root.querySelector("header");
      if (header) {
        header.textContent = title;
      }
    }
  })();
</script>

<!--
  Timeline integration:
  tl.fromTo('.hf-transition-panel-reveal', { '--hf-panel-open': 0 }, { '--hf-panel-open': 1, duration: 0.44, ease: 'power3.inOut' }, startTime);
-->