Back to Top

*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

:root {
  --bg: #050910;
  --card: #0d1117;
  --text: #f2f6ff;
  --muted: #475569;
  --border: rgba(255, 255, 255, 0.08);
  --accent: #38bdf8;
  --btn-size: 52px;
  --btn-offset: 1.75rem;
  --ease: cubic-bezier(0.4, 0, 0.2, 1);
}

body {
  font-family: Inter, system-ui, sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
}

/* ── Demo ── */
.demo {
  max-width: 640px;
  margin: 0 auto;
  padding: 3rem 1.5rem 6rem;
}

.page-header {
  margin-bottom: 3rem;
}
.demo-title {
  font-size: 1.5rem;
  font-weight: 800;
  margin-bottom: 0.375rem;
}
.demo-sub {
  color: var(--muted);
  font-size: 0.875rem;
}

/* ── Articles ── */
.articles {
  display: flex;
  flex-direction: column;
  gap: 2rem;
}

.article {
  padding: 1.5rem;
  border-radius: 14px;
  background: var(--card);
  border: 1px solid var(--border);
}

.article__title {
  font-size: 1.0625rem;
  font-weight: 700;
  margin-bottom: 0.75rem;
  letter-spacing: -0.01em;
}

.article__body {
  font-size: 0.9rem;
  color: #94a3b8;
  line-height: 1.75;
}

.article__body code {
  font-family: "SFMono-Regular", Consolas, monospace;
  font-size: 0.8125em;
  background: rgba(255, 255, 255, 0.07);
  padding: 0.1em 0.4em;
  border-radius: 4px;
  color: var(--accent);
}

/* ── Back-to-top button ── */
.btt-btn {
  position: fixed;
  bottom: var(--btn-offset);
  right: var(--btn-offset);
  width: var(--btn-size);
  height: var(--btn-size);
  display: flex;
  align-items: center;
  justify-content: center;
  background: rgba(13, 17, 23, 0.9);
  border: 1px solid var(--border);
  border-radius: 50%;
  cursor: pointer;
  color: var(--text);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  box-shadow: 0 4px 24px rgba(0, 0, 0, 0.4);

  /* Hidden by default */
  opacity: 0;
  pointer-events: none;
  transform: translateY(12px) scale(0.9);
  transition: opacity 0.3s var(--ease), transform 0.3s var(--ease), box-shadow 0.2s;

  z-index: 999;
}

.btt-btn.visible {
  opacity: 1;
  pointer-events: all;
  transform: translateY(0) scale(1);
}

.btt-btn:hover {
  box-shadow: 0 6px 32px rgba(56, 189, 248, 0.25);
  transform: translateY(-2px) scale(1.05);
}

.btt-btn:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 3px;
}

.btt-btn:active {
  transform: scale(0.96);
}

/* ── Progress ring ── */
.btt-ring {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  transform: rotate(-90deg);
  border-radius: 50%;
  overflow: visible;
}

.btt-ring__track {
  fill: none;
  stroke: rgba(255, 255, 255, 0.06);
  stroke-width: 2.5;
}

.btt-ring__fill {
  fill: none;
  stroke: var(--accent);
  stroke-width: 2.5;
  stroke-linecap: round;
  /* set dynamically by JS: stroke-dasharray and stroke-dashoffset */
  transition: stroke-dashoffset 0.1s linear;
}

/* ── Arrow icon ── */
.btt-arrow {
  position: relative;
  z-index: 1;
  color: var(--text);
  transition: transform 0.2s var(--ease);
}

.btt-btn:hover .btt-arrow {
  transform: translateY(-1px);
}

(function () {
  var THRESHOLD = 300; // px before button appears

  var btn = document.getElementById("btt-btn");
  var ringFill = document.getElementById("btt-ring-fill");

  if (!btn) return;

  // ── Progress ring setup ──
  var radius = 23; // matches SVG r attribute
  var circumference = 2 * Math.PI * radius; // ~144.51

  if (ringFill) {
    ringFill.style.strokeDasharray = circumference;
    ringFill.style.strokeDashoffset = circumference; // fully hidden
  }

  // ── Scroll handler ──
  function onScroll() {
    var scrollY = window.scrollY || window.pageYOffset;
    var maxScroll = document.documentElement.scrollHeight - window.innerHeight;
    var pct = maxScroll > 0 ? scrollY / maxScroll : 0;

    // Show / hide button
    if (scrollY > THRESHOLD) {
      btn.classList.add("visible");
    } else {
      btn.classList.remove("visible");
    }

    // Update progress ring
    if (ringFill) {
      var offset = circumference * (1 - pct);
      ringFill.style.strokeDashoffset = offset;
    }
  }

  window.addEventListener("scroll", onScroll, { passive: true });
  onScroll(); // initialise on load

  // ── Click: smooth scroll to top ──
  btn.addEventListener("click", function () {
    window.scrollTo({ top: 0, behavior: "smooth" });
  });

  // ── Keyboard activation ──
  btn.addEventListener("keydown", function (e) {
    if (e.key === "Enter" || e.key === " ") {
      e.preventDefault();
      window.scrollTo({ top: 0, behavior: "smooth" });
    }
  });
})();

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Back to Top</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>

  <!-- ── Page content (scrollable) ── -->
  <div class="demo">
    <header class="page-header">
      <h1 class="demo-title">Back to Top</h1>
      <p class="demo-sub">Scroll down to see the floating button appear. Click it to return here.</p>
    </header>

    <div class="articles">

      <article class="article">
        <h2 class="article__title">The Art of Scroll UX</h2>
        <p class="article__body">Long-form content — documentation, blog posts, landing pages — benefits enormously from a well-placed back-to-top button. Rather than forcing users to scroll all the way back, a single click returns them instantly. The key is subtlety: the button should only appear when it is needed and should never obscure content while reading.</p>
      </article>

      <article class="article">
        <h2 class="article__title">Why Position: Fixed</h2>
        <p class="article__body">The button uses <code>position: fixed</code> anchored to the bottom-right corner of the viewport. This keeps it consistently reachable regardless of scroll position, layout width, or device. A small <code>margin</code> from the edges prevents it from hugging too close to the browser chrome on mobile.</p>
      </article>

      <article class="article">
        <h2 class="article__title">Smooth Scroll Performance</h2>
        <p class="article__body">Using <code>window.scrollTo({ top: 0, behavior: 'smooth' })</code> delegates animation to the browser's native compositor, which can run off the main thread. This avoids the jank of JS-driven scroll loops while still providing a polished, animated experience across all major browsers.</p>
      </article>

      <article class="article">
        <h2 class="article__title">The Progress Ring</h2>
        <p class="article__body">This variant adds a circular SVG progress ring around the button that fills as you scroll down the page. The ring is drawn with a <code>stroke-dashoffset</code> updated on every scroll frame. The circumference of the circle is pre-calculated and stored as a CSS custom property so the math stays in JavaScript but the animation stays in CSS.</p>
      </article>

      <article class="article">
        <h2 class="article__title">Visibility Threshold</h2>
        <p class="article__body">The button appears after 300 px of scroll — roughly one viewport height on a phone. This threshold prevents the button from flashing in for short pages. You can adjust it by changing the <code>THRESHOLD</code> constant in the script. For very long pages, a higher threshold (500–800 px) may feel more appropriate.</p>
      </article>

      <article class="article">
        <h2 class="article__title">Accessibility Considerations</h2>
        <p class="article__body">The button is always in the DOM so its focus state is predictable. It uses <code>opacity: 0</code> and <code>pointer-events: none</code> when hidden — not <code>display: none</code> — which means keyboard focus can still land on it. An <code>aria-label</code> provides a meaningful name for screen readers, and the <code>tabindex</code> is left at the default <code>0</code>.</p>
      </article>

      <article class="article">
        <h2 class="article__title">Customization Tips</h2>
        <p class="article__body">All visual tokens live in CSS custom properties: button size, accent color, offset from the edges, and transition timing. Override them at the <code>:root</code> level or scope them to a parent element. The progress ring radius and stroke width are derived from <code>--btn-size</code> automatically so the ring always fits the button perfectly.</p>
      </article>

    </div>
  </div>

  <!-- ── Floating back-to-top button ── -->
  <button
    class="btt-btn"
    id="btt-btn"
    aria-label="Back to top"
    title="Back to top"
  >
    <!-- Progress ring -->
    <svg class="btt-ring" aria-hidden="true" viewBox="0 0 52 52">
      <circle class="btt-ring__track" cx="26" cy="26" r="23" />
      <circle class="btt-ring__fill" id="btt-ring-fill" cx="26" cy="26" r="23" />
    </svg>

    <!-- Arrow icon -->
    <svg class="btt-arrow" aria-hidden="true" width="18" height="18" viewBox="0 0 18 18" fill="none">
      <path d="M9 14V4M4 9l5-5 5 5" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
    </svg>
  </button>

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