Double ticker band

Build "Double ticker band" for my site using the Enrichr Library element at https://enrichr.dev/elements/double-ticker-band.html.

Read the machine-readable spec and source at https://enrichr.dev/elements/double-ticker-band.json (markup, CSS, JavaScript, parameters).
Requirements: vanilla JavaScript and CSS, no new dependencies; keep the behaviour of the source unchanged; respect prefers-reduced-motion; work on touch screens; keep it accessible.

Settings:
- top: Capture it
- bottom: Rebuild it
- separator: •
- topSpeed: 30
- bottomSpeed: 14
- topDirection: left
- bottomDirection: left
- size: 1.2
- gap: 60
- fade: 0
- topBg: #0c1e18
- topInk: #e6ff55
- bottomBg: #fbf8f1
- bottomInk: #0c1e18
- background: #fbf8f1

Verify: reduced motion shows no movement, it works at 375px wide, there are no console errors, and scrolling stays smooth.

<div class="ef-dtk">
  <div class="ef-dtk__rows" tabindex="0" role="group" aria-label="Two scrolling lines of text. Focus here to pause them.">
    <div class="ef-dtk__row ef-dtk__row--a"><p class="ef-dtk__sr"></p><div class="ef-dtk__track" aria-hidden="true"></div></div>
    <div class="ef-dtk__row ef-dtk__row--b"><p class="ef-dtk__sr"></p><div class="ef-dtk__track" aria-hidden="true"></div></div>
  </div>
</div>

/* Enrichr Library · https://enrichr.dev */
.ef-dtk { --ef-dtk-page: #fbf8f1; --ef-dtk-a-bg: #0c1e18; --ef-dtk-a-ink: #e6ff55; --ef-dtk-b-bg: #fbf8f1; --ef-dtk-b-ink: #0c1e18; --ef-dtk-size: 1.2rem; --ef-dtk-gap: 60px; --ef-dtk-fade: 0px; position: relative; width: 100%; min-height: 20rem; box-sizing: border-box; display: flex; flex-direction: column; justify-content: center; overflow: hidden; border-radius: 1rem; background: var(--ef-dtk-page); font-family: 'Parkinsans', 'Space Grotesk', sans-serif; }
.ef-dtk .ef-dtk__rows { width: 100%; overflow: hidden; outline: none; }
.ef-dtk .ef-dtk__rows:focus-visible { box-shadow: 0 0 0 3px var(--ef-dtk-a-ink), 0 0 0 6px var(--ef-dtk-a-bg); }
.ef-dtk .ef-dtk__row { position: relative; overflow: hidden; white-space: nowrap; -webkit-mask-image: var(--ef-dtk-mask, none); mask-image: var(--ef-dtk-mask, none); }
.ef-dtk .ef-dtk__row--a { padding: 12px 0; background: var(--ef-dtk-a-bg); color: var(--ef-dtk-a-ink); border-top: 3px solid var(--ef-dtk-a-bg); border-bottom: 3px solid var(--ef-dtk-a-bg); }
.ef-dtk .ef-dtk__row--b { padding: 10px 0; background: var(--ef-dtk-b-bg); color: var(--ef-dtk-b-ink); }
.ef-dtk .ef-dtk__track { display: flex; width: max-content; will-change: transform; }
.ef-dtk .ef-dtk__item { flex: none; margin: 0; padding: 0 var(--ef-dtk-gap) 0 0; font-size: var(--ef-dtk-size); font-weight: 800; line-height: 1.15; letter-spacing: 1px; text-transform: uppercase; opacity: 1; }
.ef-dtk .ef-dtk__sr { position: absolute; width: 1px; height: 1px; margin: -1px; padding: 0; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0; }

// Enrichr Library · https://enrichr.dev
// Double ticker band: two stacked full-width bands of repeated words, each on its own endless loop. Each band holds one
// short line repeated eight times as a set, the set is doubled, and the track slides by exactly half its width on a linear
// loop, so the join never shows. The top band takes 30 seconds per set and the bottom one 14, as in the original, so they
// drift past each other at different speeds; either can run the other way. On a wide screen extra sets are added and the
// loop time grows with them, so the words keep the same pace. The loops run as Web Animations on the compositor.
// Reduced motion shows the words still. The loops pause off screen, in hidden tabs, and while the band has keyboard focus.
// mount(root, { top (text), bottom (text), separator, topSpeed (s per set of 8), bottomSpeed (s per set of 8),
//               topDirection ("left" or "right"), bottomDirection, size (rem), gap (px), fade (edge fade, px),
//               topBg, topInk, bottomBg, bottomInk, background })
function mount(root, o) {
  var reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
  var box = root.querySelector(".ef-dtk");
  var group = root.querySelector(".ef-dtk__rows");
  var SET = 8; // copies per set, as in the original
  var rows = [].map.call(root.querySelectorAll(".ef-dtk__row"), function (el, i) {
    return { el: el, track: el.querySelector(".ef-dtk__track"), sr: el.querySelector(".ef-dtk__sr"), key: i ? "bottom" : "top", anim: null, dur: 0, dir: "", sig: "" };
  });
  var opts = {}, io = null, ro = null, visible = true, focused = false, destroyed = false, lastW = 0;

  function num(v, d) { v = Number(v); return isFinite(v) ? v : d; }
  function item(text) { var s = document.createElement("span"); s.className = "ef-dtk__item"; s.textContent = text; return s; }
  function frames(dir) {
    var a = { transform: "translate3d(0,0,0)" }, b = { transform: "translate3d(-50%,0,0)" };
    return dir === "right" ? [b, a] : [a, b];
  }
  function paused() { return !visible || focused || document.hidden; }

  // Start (or restart) one band's loop, keeping its current place so a live change never jumps.
  function play(r, sec, dir) {
    if (reduce || !r.track.animate) return;
    var dur = Math.max(0.5, sec) * 1000, p = 0;
    if (r.anim) {
      var ct = Number(r.anim.currentTime) || 0;
      p = r.dur ? (ct % r.dur) / r.dur : 0;
      if (r.dir && r.dir !== dir) p = 1 - p;
      r.anim.cancel();
    }
    r.anim = r.track.animate(frames(dir), { duration: dur, iterations: Infinity, easing: "linear" });
    r.anim.currentTime = p * dur;
    r.dur = dur; r.dir = dir;
    if (paused()) r.anim.pause();
  }
  // Fill a band: one set of eight must cover the width; add sets until it does, and scale the loop time to match.
  function build(r) {
    var text = String(opts[r.key] || "").trim() || " ";
    var label = opts.separator ? text + " " + opts.separator : text;
    var sig = label + "|" + opts.size + "|" + opts.gap;
    r.sr.textContent = text;
    var probe = item(label);
    r.track.textContent = "";
    r.track.appendChild(probe);
    var w = probe.getBoundingClientRect().width || 120;
    var k = Math.max(1, Math.ceil((box.clientWidth || 800) / (w * SET)));
    var n = SET * k * 2, frag = document.createDocumentFragment();
    for (var i = 0; i < n; i++) frag.appendChild(item(label));
    r.track.textContent = "";
    r.track.appendChild(frag);
    r.k = k; r.sig = sig;
  }
  function run(r) { play(r, num(opts[r.key + "Speed"], r.key === "top" ? 30 : 14) * r.k, opts[r.key + "Direction"] === "right" ? "right" : "left"); }
  function sync() { rows.forEach(function (r) { if (r.anim) { if (paused()) r.anim.pause(); else r.anim.play(); } }); }

  function update(partial) {
    opts = Object.assign({
      top: "Capture it", bottom: "Rebuild it", separator: "•", topSpeed: 30, bottomSpeed: 14, topDirection: "left", bottomDirection: "left",
      size: 1.2, gap: 60, fade: 0, topBg: "#0c1e18", topInk: "#e6ff55", bottomBg: "#fbf8f1", bottomInk: "#0c1e18", background: "#fbf8f1"
    }, opts, partial || {});
    opts.size = Math.max(0.5, num(opts.size, 1.2)); opts.gap = Math.max(0, num(opts.gap, 60)); opts.fade = Math.max(0, num(opts.fade, 0));
    var s = box.style;
    s.setProperty("--ef-dtk-page", opts.background);
    s.setProperty("--ef-dtk-a-bg", opts.topBg); s.setProperty("--ef-dtk-a-ink", opts.topInk);
    s.setProperty("--ef-dtk-b-bg", opts.bottomBg); s.setProperty("--ef-dtk-b-ink", opts.bottomInk);
    s.setProperty("--ef-dtk-size", opts.size + "rem"); s.setProperty("--ef-dtk-gap", opts.gap + "px");
    s.setProperty("--ef-dtk-mask", opts.fade > 0 && !reduce ? "linear-gradient(to right, transparent 0, #000 " + opts.fade + "px, #000 calc(100% - " + opts.fade + "px), transparent 100%)" : "none");
    if (destroyed) return;
    rows.forEach(function (r) {
      var text = String(opts[r.key] || "").trim() || " ";
      if ((opts.separator ? text + " " + opts.separator : text) + "|" + opts.size + "|" + opts.gap !== r.sig) build(r);
      run(r);
    });
    lastW = box.clientWidth;
  }
  function refit() { if (destroyed) return; rows.forEach(function (r) { r.sig = ""; build(r); run(r); }); lastW = box.clientWidth; }
  function onFocus() { focused = true; sync(); }
  function onBlur() { focused = false; sync(); }
  function onVis() { sync(); }

  update(o);
  group.addEventListener("focus", onFocus);
  group.addEventListener("blur", onBlur);
  document.addEventListener("visibilitychange", onVis);
  if ("IntersectionObserver" in window) { io = new IntersectionObserver(function (e) { visible = e[0].isIntersecting; sync(); }); io.observe(box); }
  if ("ResizeObserver" in window) {
    ro = new ResizeObserver(function () { var w = box.clientWidth; if (Math.abs(w - lastW) > 24) refit(); });
    ro.observe(box);
  }
  if (document.fonts && document.fonts.ready) document.fonts.ready.then(function () { refit(); });
  return {
    update: update,
    destroy: function () {
      destroyed = true;
      rows.forEach(function (r) { if (r.anim) r.anim.cancel(); r.anim = null; });
      group.removeEventListener("focus", onFocus); group.removeEventListener("blur", onBlur);
      document.removeEventListener("visibilitychange", onVis);
      if (io) io.disconnect();
      if (ro) ro.disconnect();
    }
  };
}

// Usage
var fx = mount(document.querySelector("#my-root"), {"top":"Capture it","bottom":"Rebuild it","separator":"•","topSpeed":30,"bottomSpeed":14,"topDirection":"left","bottomDirection":"left","size":1.2,"gap":60,"fade":0,"topBg":"#0c1e18","topInk":"#e6ff55","bottomBg":"#fbf8f1","bottomInk":"#0c1e18","background":"#fbf8f1"});