Top Loading Bar

<div class="kodza-loading-bar" role="progressbar" aria-label="Loading the page" data-links>
  <span class="kodza-loading-bar__fill"></span>
</div>

/* Kodza License: free to use, client work included. Not to resell or redistribute, ported or not: https://kodza.dev/docs/#print-license */
/* Kodza — Top Loading Bar
   The thin line at the top of the screen while a page loads. It jumps in, then creeps forward
   in shorter and shorter steps (it never claims to be done); when the page is there it runs to
   the end and slips off to the right. With data-links it starts by itself when a link leads
   to another page of the site. From your code: el.kodzaLoadingBar.start() / .done() / .set(0.4).
   Everything is tuned with the --kz-loadbar-* variables. */
.kodza-loading-bar {
  --kz-loadbar-color: #0a0a0a;
  --kz-loadbar-height: 3px;
  --kz-loadbar-ease: cubic-bezier(0.22, 1, 0.36, 1);
  --kz-loadbar-p: 0;

  position: fixed;
  top: 0;
  right: 0;
  left: 0;
  z-index: 9999;
  height: var(--kz-loadbar-height);
  overflow: hidden;
  pointer-events: none;
}

.kodza-loading-bar__fill {
  position: absolute;
  inset: 0;
  background: var(--kz-loadbar-color);
  transform: translateX(calc((var(--kz-loadbar-p) - 1) * 100%));
  transition: transform 0.5s var(--kz-loadbar-ease);
}

/* Done: runs to the end, quickly */
.kodza-loading-bar.is-done .kodza-loading-bar__fill {
  transition-duration: 0.3s;
}

/* Then slips off to the right */
.kodza-loading-bar.is-leaving .kodza-loading-bar__fill {
  transform: translateX(101%);
  transition: transform 0.55s cubic-bezier(0.65, 0, 0.35, 1);
}

/* Back to the start, unseen */
.kodza-loading-bar.is-idle .kodza-loading-bar__fill {
  transition: none;
}

@media (prefers-reduced-motion: reduce) {
  .kodza-loading-bar__fill,
  .kodza-loading-bar.is-leaving .kodza-loading-bar__fill {
    transition: none;
  }
}

/* Theming example — on a dark site:
.dark-site .kodza-loading-bar {
  --kz-loadbar-color: #f2f2f0;
}
*/

/* kodza-guard 097a3e222642 — 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, on everything the component leaves to it */

.kodza-loading-bar:not(#_),
.kodza-loading-bar__fill:not(#_) {
  box-sizing: revert;
}

// Kodza License: free to use, client work included. Not to resell or redistribute, ported or not: https://kodza.dev/docs/#print-license
// Kodza — Top Loading Bar
// start() jumps to 8% and creeps towards 90% in shrinking steps; done() runs to 100% and
// slips the line off to the right; set(p) goes to a given point. With data-links, a click on a
// link to another page of the same site starts it (and the back button resets it).
(() => {
  const SELECTOR = '.kodza-loading-bar';
  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) {
    let p = 0;
    let running = false;
    let timer = 0;

    const paint = (value) => {
      p = Math.min(1, Math.max(0, value));
      root.style.setProperty('--kz-loadbar-p', p);
      root.setAttribute('aria-valuenow', Math.round(p * 100));
    };

    // Shrinking steps: a third of what is left, then less and less
    const creep = () => {
      clearTimeout(timer);
      timer = setTimeout(() => {
        if (!running) return;
        paint(p + (0.9 - p) * (0.12 + Math.random() * 0.22));
        creep();
      }, 280 + Math.random() * 420);
    };

    const start = () => {
      clearTimeout(timer);
      root.classList.remove('is-done', 'is-leaving');
      root.classList.add('is-idle');
      paint(0);
      void root.offsetWidth;
      root.classList.remove('is-idle');
      root.setAttribute('aria-busy', 'true');
      running = true;
      paint(0.08);
      creep();
    };

    const done = () => {
      if (!running) return;
      running = false;
      clearTimeout(timer);
      root.removeAttribute('aria-busy');
      root.classList.add('is-done');
      paint(1);
      timer = setTimeout(() => {
        root.classList.add('is-leaving');
        timer = setTimeout(() => {
          root.classList.add('is-idle');
          root.classList.remove('is-done', 'is-leaving');
          paint(0);
        }, reduceMotion ? 0 : 560);
      }, reduceMotion ? 200 : 320);
    };

    root.kodzaLoadingBar = { start, done, set: (value) => (running ? paint(value) : (start(), paint(value))) };
    // Stopped mid-load: the creeping and the exit stop with it
    signal.addEventListener('abort', () => {
      clearTimeout(timer);
      running = false;
      delete root.kodzaLoadingBar;
    });
    root.classList.add('is-idle');
    paint(0);

    if (root.hasAttribute('data-links')) {
      document.addEventListener('click', (event) => {
        const link = event.target.closest?.('a[href]');
        if (!link || event.defaultPrevented || event.button !== 0 || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
        if (link.target && link.target !== '_self') return;
        if (link.hasAttribute('download')) return;
        const url = new URL(link.href, location.href);
        // Another page of this site (a #section of the same page loads nothing)
        if (url.origin !== location.origin || (url.pathname === location.pathname && url.search === location.search)) return;
        start();
      }, { signal });
      window.addEventListener('pageshow', (event) => event.persisted && done(), { signal });
    }
  }

  // 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 });
})();

'use client';
// Kodza — Top Loading Bar, for React. From https://kodza.dev/components/top-loading-bar/
// 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: top-loading-bar.css and top-loading-bar.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 './top-loading-bar.css';

export type TopLoadingBarProps = Omit<ComponentPropsWithoutRef<'div'>, "children" | "vars" | "links"> & {
  /** data-links on the root */
  links?: string;
  /** Settings, as CSS variables on the root */
  vars?: {
    /** default: #0a0a0a */
    "--kz-loadbar-color"?: string | number;
    /** default: 3px */
    "--kz-loadbar-height"?: string | number;
    /** default: cubic-bezier(0.22, 1, 0.36, 1) */
    "--kz-loadbar-ease"?: string | number;
  };
  className?: string;
};

export function TopLoadingBar({ links, vars, className, style, ...rest }: TopLoadingBarProps) {
  const ref = useRef<ComponentRef<'div'>>(null);
  const classes = ["kodza-loading-bar", 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, links, vars, 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('./top-loading-bar.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 role="progressbar" aria-label="Loading the page" key={key} {...rest} className={classes} data-links={links} style={{ ...vars, ...style } as CSSProperties} ref={ref}>
      <span className="kodza-loading-bar__fill" />
    </div>
  );
}