Reading Progress Highlight.

/* Reading Progress Highlight — A thin progress rule tracks how far the reader has moved. */
.reading-progress-highlight {
  --dur: scroll-linked;
  --ease: linear;
  --stagger: 60ms;
  will-change: transform, opacity;
}

.reading-progress-highlight > * {
  opacity: 0;
  transform: translateY(14px);
  animation: reading-progress-highlight-in var(--dur) var(--ease) forwards;
  animation-delay: calc(var(--i, 0) * var(--stagger));
}

/* scaled to 0 on the X axis -> scaling the bar to match scroll progress */
@keyframes reading-progress-highlight-in {
  to {
    opacity: 1;
    transform: none;
    filter: none;
  }
}

@media (prefers-reduced-motion: reduce) {
  .reading-progress-highlight > * {
    opacity: 1;
    transform: none;
    animation: none;
  }
}

import { useEffect, useRef } from "react";

/** Reading Progress Highlight — trigger: scroll position within the article */
export function ReadingProgressHighlight({ children }: { children: React.ReactNode }) {
  const ref = useRef<HTMLDivElement>(null);

  useEffect(() => {
    const el = ref.current;
    if (!el) return;
    if (window.matchMedia("(prefers-reduced-motion: reduce)").matches) {
      el.dataset["state"] = "in";
      return;
    }
    const io = new IntersectionObserver(
      ([entry]) => {
        if (entry?.isIntersecting) {
          el.dataset["state"] = "in";
          io.unobserve(el);
        }
      },
      { threshold: 0.2 },
    );
    io.observe(el);
    return () => io.disconnect();
  }, []);

  return (
    <div ref={ref} className="reading-progress-highlight" data-state="out">
      {children}
    </div>
  );
}