Sliding tab pill

"use client";

import { useEffect, useId, useRef } from "react";

/**
 * Segmented tab bar with a pill that slides to the selected label.
 *
 * State is radio inputs, not React state, so the control works before hydration
 * and stays keyboard-navigable for free. The effect only measures the checked
 * label and drives the pill; it never owns which tab is selected.
 */
export function TabPillBar({
  tabs,
  onSelect,
}: {
  tabs: { id: string; label: string }[];
  onSelect?: (id: string) => void;
}) {
  const pillRef = useRef<HTMLSpanElement>(null);
  const name = useId();

  useEffect(() => {
    const pill = pillRef.current;
    const bar = pill?.parentElement;
    if (!pill || !bar) return;

    const move = (animate: boolean) => {
      const selected = bar.querySelector<HTMLElement>("input:checked + [data-tab]");
      if (!selected) return;

      // Kill the transition for un-animated moves (mount, resize), otherwise the
      // pill visibly flies in from 0,0 on first paint.
      if (!animate) {
        pill.style.transition = "none";
        pill.getBoundingClientRect();
      }

      pill.style.transform = `translate(${selected.offsetLeft}px, ${selected.offsetTop}px)`;
      pill.style.width = `${selected.offsetWidth}px`;
      pill.style.height = `${selected.offsetHeight}px`;

      if (!animate) {
        pill.getBoundingClientRect();
        pill.style.transition = "";
      }
    };

    move(false);
    // Signals CSS that the pill is measured and can take over the selected state.
    bar.dataset.pill = "on";

    const onChange = (e: Event) => {
      move(true);
      const target = e.target as HTMLInputElement;
      if (target?.value) onSelect?.(target.value);
    };
    bar.addEventListener("change", onChange);

    const observer = new ResizeObserver(() => move(false));
    observer.observe(bar);

    return () => {
      bar.removeEventListener("change", onChange);
      observer.disconnect();
      delete bar.dataset.pill;
    };
  }, [onSelect]);

  return (
    <div className="t-tabs">
      <span ref={pillRef} className="t-tabs-pill" aria-hidden />
      {tabs.map((tab, i) => (
        <div key={tab.id} className="contents">
          <input
            type="radio"
            name={name}
            id={`${name}-${tab.id}`}
            value={tab.id}
            defaultChecked={i === 0}
            className="peer sr-only"
          />
          {/* `peer`, not `has-[:checked]` — the input is a sibling of the label,
              not a descendant, so has-* never matches and the selected label
              stays grey on the white pill. */}
          <label
            htmlFor={`${name}-${tab.id}`}
            data-tab={tab.id}
            className="flex min-h-[44px] cursor-pointer items-center rounded-iq-core border border-transparent px-4 text-iq-small text-iq-ink-muted transition-colors duration-200 hover:text-iq-ink peer-checked:text-iq-canvas sm:min-h-0 sm:whitespace-nowrap sm:rounded-full sm:py-2"
          >
            {tab.label}
          </label>
        </div>
      ))}
    </div>
  );
}