# Back to Top

> A round button that shows up a screen down, its ring filling as you read. Its label unrolls on hover, the arrow takes off when you click.

Scroll · scroll, button, progress, top · HTML, CSS, vanilla JS · no dependencies · updated 2026-10-16
Demo: https://kodza.dev/components/back-to-top/

Takes the whole screen (or reacts to the page scroll): give it a page, not a small box.

## Files

The code below, served as files: byte for byte what the Kodza site copies (the CSS includes its theme guard).

- [`kodza-back-to-top.css`](https://kodza.dev/registry/back-to-top/kodza-back-to-top.css) · 8.8 KB
- [`kodza-back-to-top.js`](https://kodza.dev/registry/back-to-top/kodza-back-to-top.js) · 3.6 KB
- [`kodza-back-to-top.html`](https://kodza.dev/registry/back-to-top/kodza-back-to-top.html) · 0.5 KB · the markup, to adapt and paste into your page

**Can you run shell commands? Download the CSS and the JS, don't retype them.** Retyping costs thousands of tokens and can drop or change lines of the CSS. Put the files where your stack expects them (Use it says where: change the `-o` paths):

```bash
curl -fsSL --create-dirs -o kodza/kodza-back-to-top.css https://kodza.dev/registry/back-to-top/kodza-back-to-top.css
curl -fsSL --create-dirs -o kodza/kodza-back-to-top.js https://kodza.dev/registry/back-to-top/kodza-back-to-top.js
```

Then load the files from your page as Use it shows, instead of pasting the content. The HTML is the part you adapt (text, links, modifiers): paste it from the Code section. Copy the CSS and the JS from the Code section only if you can't run commands, exactly as they are. (Windows PowerShell: `curl.exe`.)

## API

- Root: `.kodza-top`
- Variables (set them on the root element itself: it declares its own defaults):
  - `--kz-top-bg: #ffffff`
  - `--kz-top-fg: #0a0a0a`
  - `--kz-top-line: rgba(10, 10, 10, 0.1)`
  - `--kz-top-shadow: 0 10px 30px -10px rgba(10, 10, 10, 0.3)`
  - `--kz-top-size: 48px`
  - `--kz-top-gap: 24px`
  - `--kz-top-ease: cubic-bezier(0.22, 1, 0.36, 1)`
  - `--kz-top-spring: cubic-bezier(0.34, 1.56, 0.64, 1)`
- Lifecycle: `kodza:init` / `kodza:destroy` on `document`, with `{ detail: { root } }` to target a container.

## Dark mode

The values Kodza uses on a dark background (`#0a0a0a`). For a page that follows the system theme, add them after the component's CSS:

```css
@media (prefers-color-scheme: dark) {
  .kodza-top {
    --kz-top-bg: #f2f2f0;
    --kz-top-fg: #0a0a0a;
    --kz-top-shadow: 0 10px 30px -10px rgba(0, 0, 0, 0.8);
  }
}
```

A site with its own theme switch puts the same rules under its dark selector instead (`[data-theme="dark"] .kodza-top { … }`); a single dark section, under that section's class.

## Use it

### Plain HTML page

- CSS: `kodza/kodza-back-to-top.css` next to the page (the Files commands as they are), linked in `<head>`: `<link rel="stylesheet" href="kodza/kodza-back-to-top.css">`. Without a shell, paste it in a `<style>` in `<head>` instead. It only touches `kodza-` classes.
- HTML: where the component goes. Paste it as many times as you need: instances never interfere.
- JS: `kodza/kodza-back-to-top.js`, loaded with `<script src="kodza/kodza-back-to-top.js" defer></script>` in `<head>`. Without a shell, paste it in a `<script>` before `</body>` (or in `<head>`: it waits for the page). One copy per page, whatever the number of instances.
- Restyle with the `--kz-…` variables, set on the component's root element itself (the root declares its own defaults, so a value on an ancestor is ignored), and with the modifiers / attributes listed under API.
- Font: it takes the page's own. The Kodza look is Geist: `<link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Geist:wght@400;500;600;700&family=Geist+Mono:wght@400;500&display=swap">`.
- Content added later (AJAX navigation, a SPA route, a builder preview): start it with `document.dispatchEvent(new CustomEvent('kodza:init', { detail: { root: container } }))`; before removing it, the same call with `'kodza:destroy'` stops it and puts the original markup back.

### Claude artifact (claude.ai)

- Paste the CSS in a `<style>` and the JS in a `<script>` at the end, in the artifact itself: it loads stylesheets only from Google Fonts and scripts only from a few CDNs, so no `<link>` or `<script src>` to Kodza.
- Writing the artifact file with a shell? Insert the downloaded files (Files) with a command instead of retyping them: a line `KODZA-CSS` alone inside the `<style>`, `KODZA-JS` inside the `<script>`, then `sed -e '/^KODZA-CSS$/{r kodza/kodza-back-to-top.css' -e 'd' -e '}' -e '/^KODZA-JS$/{r kodza/kodza-back-to-top.js' -e 'd' -e '}' page.html > artifact.html`.
- Font: load Geist from Google Fonts (allowed): `<link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Geist:wght@400;500;600;700&family=Geist+Mono:wght@400;500&display=swap">`.
- Dark mode: the viewer sets `data-theme="dark"` or `"light"` on `<html>`, or nothing when it follows the system. Write the Dark mode rules twice: inside `@media (prefers-color-scheme: dark)` with `:root:not([data-theme="light"])` in front of each selector, and with `:root[data-theme="dark"]` in front of each selector.

### WordPress

- HTML: a **Custom HTML** block (or a pattern / template part). WordPress keeps this markup as it is only for users allowed to post unfiltered HTML (Administrators; Super Admins on multisite): for anyone else it strips tags such as `<svg>`, `<input>` or `<dialog>`, so add it from the theme (a pattern or template part) instead.
- CSS and JS: the downloaded files go in the theme, under `wp-content/themes/<your-theme>/kodza/` (the child theme if there is one), enqueued from its functions.php (or a code-snippets plugin):
  ```php
  add_action('wp_enqueue_scripts', function () {
    $dir = get_stylesheet_directory() . '/kodza/';
    $uri = get_stylesheet_directory_uri() . '/kodza/';
    wp_enqueue_style('kodza-back-to-top', $uri . 'kodza-back-to-top.css', [], filemtime($dir . 'kodza-back-to-top.css'));
    wp_enqueue_script('kodza-back-to-top', $uri . 'kodza-back-to-top.js', [], filemtime($dir . 'kodza-back-to-top.js'), true);
  });
  ```
  Scoped to `kodza-` classes, the CSS never touches the theme. One copy of the JS, whatever the number of blocks.
- No access to the theme files? The CSS can also go, as it is, in **Appearance → Customize → Additional CSS** (classic theme) or **Styles → Additional CSS** in the Site Editor (block theme).
- Theme with AJAX page transitions: fire `kodza:init` on the new content after each swap (and `kodza:destroy` on the old one).
- Performance plugin (WP Rocket, LiteSpeed Cache, Perfmatters…) with **Delay JavaScript execution** turned on: add `kodza` to the scripts it must not delay. Otherwise the component waits for the visitor's first scroll, tap or mouse move, and that first click can be lost.
- Colours: by default, match the theme's palette. After the component's CSS, point these variables at the theme's colours, on the component root, Kodza's value as the fallback:
  `.kodza-top { --kz-top-fg: var(TEXT, #0a0a0a); --kz-top-bg: var(BACKGROUND, #ffffff); }`
  TEXT, BACKGROUND and ACCENT (the colour the theme fills its own buttons with) are the theme's own colour variables — block themes (theme.json, like Twenty Twenty-Five): `--wp--preset--color--contrast`, `--wp--preset--color--base`, `--wp--preset--color--contrast` (they fill their buttons with it; their accent-1… colours are decorative); Astra: `--ast-global-color-2`, `--ast-global-color-4`, `--ast-global-color-0`; Kadence: `--global-palette3`, `--global-palette9`, `--global-palette1`; GeneratePress: `--contrast`, `--base-3`, `--accent`; Blocksy: `--theme-palette-color-4`, `--theme-palette-color-8`, `--theme-palette-color-1`. Use only these names, and only if the site really has them (the theme's CSS, or Styles → Colors): the `--wp--preset--color--black`, `--white`, `--vivid-…` that WordPress adds everywhere are not the theme's palette. Another theme, or one without these variables (Hello Elementor, OceanWP): leave the variables alone, it keeps Kodza's monochrome; same when the developer asks for monochrome or the Kodza look. Leave the translucent greys (`rgba(10, 10, 10, …)`) as they are: they suit any light background. On a dark palette, set the other variables from Dark mode too.
- WordPress turns straight quotes and `--` into typographic ones in the text of a Custom HTML block (not inside `<code>` or `<pre>`).

#### With Elementor

- HTML: an **HTML** widget (Elementor shows it to administrators only). It keeps the code exactly as pasted, straight quotes included.
- CSS and JS: as above. Elementor's Site Settings → Theme Style styles every button, field, link and image of the page, components included; the theme guard at the end of the CSS keeps the component's buttons, fields and link colours as they are.
- The Elementor editor redraws each widget after the page has loaded: without the script below, the component stays still in the editor preview (the published page works). Add it once per site, in the footer like the component's script (`kodza/elementor-editor.js`, or a code-snippets plugin):

```js
// Kodza in the Elementor editor: the editor redraws each HTML widget after the page has loaded.
// Start Kodza in what it draws, stop it in what it replaces or removes. Once per site.
window.addEventListener('elementor/frontend/init', () => {
  if (!elementorFrontend.isEditMode()) return;
  class KodzaWidget extends elementorModules.frontend.handlers.Base {
    onInit() {
      super.onInit();
      this.kodza = [...this.$element[0].children];
      document.dispatchEvent(new CustomEvent('kodza:init', { detail: { root: this.$element[0] } }));
    }
    onDestroy() {
      super.onDestroy();
      this.kodza.forEach((root) => document.dispatchEvent(new CustomEvent('kodza:destroy', { detail: { root } })));
    }
  }
  elementorFrontend.elementsHandler.attachHandler('html', KodzaWidget);
});
```

- No child theme: everything can go in the HTML widget instead, `<style>` with the CSS, the HTML, then `<script>` with the JS. It works on the page and in the editor without the script above.

### Shopify

- Files: `assets/kodza-back-to-top.css` and `assets/kodza-back-to-top.js` (the Files commands with `-o assets/…`).
- In the section: `{{ 'kodza-back-to-top.css' | asset_url | stylesheet_tag }}` and `<script src="{{ 'kodza-back-to-top.js' | asset_url }}" defer></script>`, then the HTML.
- The theme editor re-renders sections: restart them with
  `document.addEventListener('shopify:section:load', (e) => document.dispatchEvent(new CustomEvent('kodza:init', { detail: { root: e.target } })));`
  and the same with `shopify:section:unload` → `kodza:destroy`.

### React / Next.js

- Files, side by side (e.g. in `components/kodza/`): `BackToTop.tsx` (below), the CSS block under Code saved as `back-to-top.css` and the JS block as `back-to-top.js`, both as they are. The HTML block is already inside the component: skip it.
- Use it: `import { BackToTop } from '@/components/kodza/BackToTop';` then `<BackToTop>Back to top</BackToTop>`. Props: `vars` (the `--kz-…` variables listed under API, e.g. `vars={{ '--kz-top-bg': … }}`), `children` (the text, plain), `className`, `style` and the other props of its `<div>` (`onClick`, `aria-…`).
- Next.js App Router: it is a Client Component already (`'use client'`). Pages Router: Next.js takes global CSS only in `pages/_app`, so move the `import './back-to-top.css'` line there.
- Safe under StrictMode, client-side navigation and prop changes: when a prop changes, the `key` gives React a fresh element, `kodza:destroy` stops the script on the old one and `kodza:init` starts it on the new one.

```tsx
'use client';
// Kodza — Back to Top, for React. From https://kodza.dev/components/back-to-top/
// Kodza License: free to use, client work included. Not to resell or redistribute, ported or not: https://kodza.dev/docs/#print-license
// Next to this file: back-to-top.css and back-to-top.js, the component's CSS and JS from Kodza, as they are.
// Yours to edit: the markup is plain JSX below.
// The script changes this markup once React has rendered it (it adds layers, splits text, moves
// nodes), so React must not patch it afterwards. When a prop changes, the key gives React a fresh
// element: kodza:destroy stops the script on the old one, kodza:init starts it on the new one.
import { useRef, useEffect, type ComponentPropsWithoutRef, type ComponentRef, type CSSProperties } from 'react';
import './back-to-top.css';

export type BackToTopProps = Omit<ComponentPropsWithoutRef<'div'>, "children" | "vars"> & {
  /** Settings, as CSS variables on the root */
  vars?: {
    /** default: #ffffff */
    "--kz-top-bg"?: string | number;
    /** default: #0a0a0a */
    "--kz-top-fg"?: string | number;
    /** default: rgba(10, 10, 10, 0.1) */
    "--kz-top-line"?: string | number;
    /** default: 0 10px 30px -10px rgba(10, 10, 10, 0.3) */
    "--kz-top-shadow"?: string | number;
    /** default: 48px */
    "--kz-top-size"?: string | number;
    /** default: 24px */
    "--kz-top-gap"?: string | number;
    /** default: cubic-bezier(0.22, 1, 0.36, 1) */
    "--kz-top-ease"?: string | number;
    /** default: cubic-bezier(0.34, 1.56, 0.64, 1) */
    "--kz-top-spring"?: string | number;
  };
  /** The text, plain: the script reads it and rebuilds the markup around it */
  children?: string;
  className?: string;
};

export function BackToTop({ vars, children = "Back to top", className, style, ...rest }: BackToTopProps) {
  const ref = useRef<ComponentRef<'div'>>(null);
  const classes = ["kodza-top", className].filter(Boolean).join(' ');
  // Everything the markup is made of: when it changes, React gets a fresh element (see the top)
  const key = JSON.stringify([classes, vars, children, style, rest]);

  // Starts the script on this element once React has rendered it, stops it when it goes
  useEffect(() => {
    const root = ref.current;
    let live = true;
    // @ts-ignore: a plain script, no types to find
    import('./back-to-top.js').then(() => {
      if (live) document.dispatchEvent(new CustomEvent('kodza:init', { detail: { root } }));
    });
    return () => {
      live = false;
      document.dispatchEvent(new CustomEvent('kodza:destroy', { detail: { root } }));
    };
  }, [key]);

  return (
    <div key={key} {...rest} className={classes} style={{ ...vars, ...style } as CSSProperties} ref={ref}>
      <button className="kodza-top__button" type="button" aria-label="Back to top">
        <svg className="kodza-top__ring" viewBox="0 0 48 48" aria-hidden="true">
          <circle className="kodza-top__track" cx="24" cy="24" r="22.5" />
          <circle className="kodza-top__fill" cx="24" cy="24" r="22.5" pathLength="1" />
        </svg>
        {' '}
        <span className="kodza-top__arrow" aria-hidden="true">
          <svg viewBox="0 0 16 16">
            <path d="M8 13V3M3.5 7.5 8 3l4.5 4.5" />
          </svg>
        </span>
        {' '}
        <span className="kodza-top__label" aria-hidden="true">{children}</span>
      </button>
    </div>
  );
}
```

## Code

```html
<div class="kodza-top">
  <button class="kodza-top__button" type="button" aria-label="Back to top">
    <svg class="kodza-top__ring" viewBox="0 0 48 48" aria-hidden="true"><circle class="kodza-top__track" cx="24" cy="24" r="22.5"/><circle class="kodza-top__fill" cx="24" cy="24" r="22.5" pathLength="1"/></svg>
    <span class="kodza-top__arrow" aria-hidden="true"><svg viewBox="0 0 16 16"><path d="M8 13V3M3.5 7.5 8 3l4.5 4.5"/></svg></span>
    <span class="kodza-top__label" aria-hidden="true">Back to top</span>
  </button>
</div>
```

```css
/* Kodza License: free to use, client work included. Not to resell or redistribute, ported or not: https://kodza.dev/docs/#print-license */
/* Kodza — Back to Top
   A round button that shows up once the visitor is a screen down, its ring filling as they
   read. On hover its label unrolls beside it and the arrow lifts; on click the arrow takes off
   and comes back from below while the page glides up. Place it at the end of the page: it
   stays at the bottom of the screen (position: sticky), on the right.
   Everything is tuned with the --kz-top-* variables. */
.kodza-top {
  --kz-top-bg: #ffffff;
  --kz-top-fg: #0a0a0a;
  --kz-top-line: rgba(10, 10, 10, 0.1);
  --kz-top-shadow: 0 10px 30px -10px rgba(10, 10, 10, 0.3);
  --kz-top-size: 48px;
  --kz-top-gap: 24px;
  --kz-top-ease: cubic-bezier(0.22, 1, 0.36, 1);
  --kz-top-spring: cubic-bezier(0.34, 1.56, 0.64, 1);

  position: sticky;
  bottom: var(--kz-top-gap);
  z-index: 40;
  display: flex;
  box-sizing: border-box;
  justify-content: flex-end;
  height: 0;
  padding-right: var(--kz-top-gap);
  pointer-events: none;
}

.kodza-top__button {
  position: relative;
  display: grid;
  place-items: center;
  width: var(--kz-top-size);
  height: var(--kz-top-size);
  padding: 0;
  border: 0;
  border-radius: 50%;
  background: var(--kz-top-bg);
  box-shadow: var(--kz-top-shadow);
  color: var(--kz-top-fg);
  cursor: pointer;
  opacity: 0;
  transform: translateY(calc(-100% + 12px)) scale(0.6);
  visibility: hidden;
  transition:
    opacity 0.25s ease,
    transform 0.35s ease-in,
    visibility 0s linear 0.35s;
}

/* Shown once the visitor is a screen down */
.kodza-top.is-shown .kodza-top__button {
  opacity: 1;
  transform: translateY(-100%);
  visibility: visible;
  pointer-events: auto;
  transition:
    opacity 0.3s ease,
    transform 0.6s var(--kz-top-spring),
    visibility 0s;
}

.kodza-top__button:focus-visible {
  outline: 2px solid var(--kz-top-fg);
  outline-offset: 3px;
}

/* The ring fills with the page */
.kodza-top__ring {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  transform: rotate(-90deg);
}

.kodza-top__ring circle {
  fill: none;
  stroke-width: 2;
}

.kodza-top__track {
  stroke: var(--kz-top-line);
}

.kodza-top__fill {
  stroke: var(--kz-top-fg);
  stroke-dasharray: 1;
  stroke-dashoffset: calc(1 - var(--kz-top-progress, 0));
  stroke-linecap: round;
}

/* The arrow: lifts on hover, takes off on click and comes back from below */
.kodza-top__arrow {
  display: grid;
  width: 16px;
  height: 16px;
  overflow: hidden;
}

.kodza-top__arrow svg {
  grid-area: 1 / 1;
  width: 16px;
  height: 16px;
  fill: none;
  stroke: currentColor;
  stroke-width: 1.8;
  stroke-linecap: round;
  stroke-linejoin: round;
  transition: transform 0.4s var(--kz-top-ease);
}

.kodza-top__button:hover .kodza-top__arrow svg {
  transform: translateY(-2px);
}

.kodza-top.is-flying .kodza-top__arrow svg {
  animation: kodza-top-fly 0.7s var(--kz-top-ease);
}

@keyframes kodza-top-fly {
  45% {
    transform: translateY(-120%);
  }

  46% {
    transform: translateY(120%);
  }

  100% {
    transform: none;
  }
}

/* The label unrolls beside the button, from the button outwards */
.kodza-top__label {
  position: absolute;
  top: 50%;
  right: calc(100% + 8px);
  padding: 8px 14px;
  border-radius: 999px;
  background: var(--kz-top-fg);
  color: var(--kz-top-bg);
  font-family: ui-monospace, "SF Mono", Menlo, monospace;
  font-size: 12px;
  white-space: nowrap;
  clip-path: inset(0 0 0 100% round 999px);
  transform: translate(8px, -50%);
  transition:
    clip-path 0.3s var(--kz-top-ease),
    transform 0.3s var(--kz-top-ease);
  pointer-events: none;
}

.kodza-top__button:hover .kodza-top__label,
.kodza-top__button:focus-visible .kodza-top__label {
  clip-path: inset(0 0 0 0 round 999px);
  transform: translate(0, -50%);
  transition-duration: 0.45s;
}

@media (hover: none) {
  .kodza-top__label {
    display: none;
  }
}

@media (prefers-reduced-motion: reduce) {
  .kodza-top__button,
  .kodza-top.is-shown .kodza-top__button,
  .kodza-top__arrow svg,
  .kodza-top__label {
    transition-duration: 0.01ms !important;
  }

  .kodza-top.is-flying .kodza-top__arrow svg {
    animation: none;
  }
}

/* Theming example — on a dark background:
.dark-section .kodza-top {
  --kz-top-bg: #f2f2f0;
  --kz-top-fg: #0a0a0a;
  --kz-top-shadow: 0 10px 30px -10px rgba(0, 0, 0, 0.8);
}
*/

/* kodza-guard fdde43541006 — generated by Kodza, do not edit.
   Theme guard: keeps a site's own CSS (WordPress themes, Bootstrap, CSS resets…) off this
   component. :not(#_) is always true and weighs as much as an id, so these lines win over
   theme rules made of classes and tags. Customise the component through its --kz-* variables. */

/* What the browser gives, where the component sets it only in some states: at the lowest weight, so its own rules below put it back */

:where(.kodza-top__button):not(#_) {
  outline: revert;
  outline-offset: revert;
}

/* What the browser gives, on everything the component leaves to it */

.kodza-top__button:not(#_) {
  margin: revert;
  text-decoration: revert;
  list-style: revert;
  box-sizing: revert;
  float: revert;
  appearance: revert;
  max-width: revert;
  max-height: revert;
  font-family: revert;
  font-size: revert;
  font-weight: revert;
  font-style: revert;
  line-height: revert;
  letter-spacing: revert;
  text-transform: revert;
  text-underline-offset: revert;
  text-align: revert;
  text-indent: revert;
  text-shadow: revert;
  vertical-align: revert;
  white-space: revert;
  min-height: revert;
}

.kodza-top__ring:not(#_) {
  border: revert;
  margin: revert;
  padding: revert;
  border-radius: revert;
  outline: revert;
  text-decoration: revert;
  transition: revert;
  list-style: revert;
  box-sizing: revert;
  float: revert;
  background-color: revert;
  background-image: revert;
  box-shadow: revert;
  outline-offset: revert;
  appearance: revert;
  max-width: revert;
  max-height: revert;
  color: revert;
  font-family: revert;
  font-size: revert;
  font-weight: revert;
  font-style: revert;
  line-height: revert;
  letter-spacing: revert;
  text-transform: revert;
  text-underline-offset: revert;
  text-align: revert;
  text-indent: revert;
  text-shadow: revert;
  vertical-align: revert;
}

.kodza-top__track:not(#_),
.kodza-top__fill:not(#_),
.kodza-top__arrow:not(#_),
.kodza-top__label:not(#_) {
  box-sizing: revert;
}

.kodza-top__arrow svg:where(:not([class*="kodza-"])):not(#_) {
  border: revert;
  margin: revert;
  padding: revert;
  border-radius: revert;
  outline: revert;
  text-decoration: revert;
  list-style: revert;
  box-sizing: revert;
  float: revert;
  position: revert;
  background-color: revert;
  background-image: revert;
  box-shadow: revert;
  outline-offset: revert;
  appearance: revert;
  max-width: revert;
  max-height: revert;
  color: revert;
  font-family: revert;
  font-size: revert;
  font-weight: revert;
  font-style: revert;
  line-height: revert;
  letter-spacing: revert;
  text-transform: revert;
  text-underline-offset: revert;
  text-align: revert;
  text-indent: revert;
  text-shadow: revert;
  vertical-align: revert;
}

.kodza-top__ring:not(#_):where(:not([hidden])),
.kodza-top__arrow svg:where(:not([class*="kodza-"])):not(#_):where(:not([hidden])) {
  display: revert;
}

.kodza-top__ring:not(#_):where([hidden]),
.kodza-top__arrow svg:where(:not([class*="kodza-"])):not(#_):where([hidden]) {
  display: none;
}

/* The component's own rules, one step stronger: real buttons and fields, what it sets on tags themes style (links, lists, headings, images…), and the root width (a theme may narrow it, never widen it) */

.kodza-top__button:not(#_) {
  position: relative;
  display: grid;
  place-items: center;
  width: var(--kz-top-size);
  height: var(--kz-top-size);
  padding: 0;
  border: 0;
  border-radius: 50%;
  background: var(--kz-top-bg);
  box-shadow: var(--kz-top-shadow);
  color: var(--kz-top-fg);
  cursor: pointer;
  opacity: 0;
  transform: translateY(calc(-100% + 12px)) scale(0.6);
  visibility: hidden;
  transition: opacity 0.25s ease,
    transform 0.35s ease-in,
    visibility 0s linear 0.35s;
}

.kodza-top.is-shown .kodza-top__button:not(#_) {
  opacity: 1;
  transform: translateY(-100%);
  visibility: visible;
  pointer-events: auto;
  transition: opacity 0.3s ease,
    transform 0.6s var(--kz-top-spring),
    visibility 0s;
}

.kodza-top__button:focus-visible:not(#_) {
  outline: 2px solid var(--kz-top-fg);
  outline-offset: 3px;
}

.kodza-top__ring:not(#_) {
  position: absolute;
}

.kodza-top__arrow svg:not(#_) {
  transition: transform 0.4s var(--kz-top-ease);
}

@media (prefers-reduced-motion: reduce) {
  .kodza-top__button:not(#_),
  .kodza-top.is-shown .kodza-top__button:not(#_) {
    transition-duration: 0.01ms !important;
  }
}

@media (prefers-reduced-motion: reduce) {
  .kodza-top__arrow svg:not(#_) {
    transition-duration: 0.01ms !important;
  }
}
```

```js
// Kodza License: free to use, client work included. Not to resell or redistribute, ported or not: https://kodza.dev/docs/#print-license
// Kodza — Back to Top
// Shows the button once the page has scrolled most of a screen (and while it rides the bottom
// of the screen), fills its ring with the
// reading progress, and glides back to the top on click (the arrow takes off and lands).
(() => {
  const SELECTOR = '.kodza-top';
  const reduceMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // One instance. Everything it starts is tied to `signal`, so kodza:destroy stops it all
  function mount(root, signal) {
    const update = () => {
      const max = document.documentElement.scrollHeight - innerHeight;
      const progress = max > 0 ? Math.min(1, window.scrollY / max) : 0;
      const far = window.scrollY > innerHeight * 0.8;
      root.style.setProperty('--kz-top-progress', progress.toFixed(4));
      // Only while it rides the bottom of the screen: resting at the end of its section
      // (mid-screen), it steps aside
      const gap = parseFloat(getComputedStyle(root).bottom) || 0;
      const top = root.getBoundingClientRect().top;
      const riding = top >= innerHeight - gap - 2 && top <= innerHeight + 2;
      root.classList.toggle('is-shown', far && riding);
    };

    const button = root.querySelector('.kodza-top__button');
    button?.addEventListener('click', () => {
      root.classList.remove('is-flying');
      void root.offsetWidth;
      root.classList.add('is-flying');
      window.scrollTo({ top: 0, behavior: reduceMotion ? 'auto' : 'smooth' });
    }, { signal });
    root.addEventListener('animationend', () => root.classList.remove('is-flying'), { signal });

    let ticking = false;
    window.addEventListener('scroll', () => {
      if (ticking) return;
      ticking = true;
      requestAnimationFrame(() => {
        ticking = false;
        if (!signal.aborted) update();
      });
    }, { passive: true, signal });
    window.addEventListener('resize', update, { signal });
    update();
  }

  // Kodza lifecycle, the same in every component: `kodza:init` starts the instances inside
  // event.detail.root (the whole page by default), `kodza:destroy` stops them and puts their
  // markup back. For themes and apps that swap content without a page load.
  const instances = new WeakMap();
  const find = (scope = document) => [scope, ...scope.querySelectorAll(SELECTOR)].filter((el) => el.matches?.(SELECTOR));
  const init = (scope) => find(scope).forEach((root) => {
    if (root.classList.contains('is-ready')) return; // already running, maybe from another copy of this script
    const controller = new AbortController();
    instances.set(root, { controller, html: root.innerHTML, attrs: [...root.attributes].map((a) => [a.name, a.value]) });
    mount(root, controller.signal);
    root.classList.add('is-ready');
  });
  const destroy = (scope) => find(scope).forEach((root) => {
    const instance = instances.get(root);
    if (!instance) return;
    instances.delete(root);
    instance.controller.abort();
    [...root.attributes].forEach((a) => root.removeAttribute(a.name));
    instance.attrs.forEach(([name, value]) => root.setAttribute(name, value));
    root.innerHTML = instance.html;
  });
  document.addEventListener('kodza:init', (event) => init(event.detail?.root));
  document.addEventListener('kodza:destroy', (event) => destroy(event.detail?.root));
  init();
  // Loaded in <head>, the script runs before the markup exists: once more when it is parsed
  if (document.readyState === 'loading') document.addEventListener('DOMContentLoaded', () => init(), { once: true });
})();
```

Free to use, from Kodza (https://kodza.dev).