Back to Top

- One effect subscribes to `scroll` on the container (or window) plus
  `resize`, and reads once on the next frame. Progress goes into a motion
  value (no re-render per scroll); `visible` is state and only changes when
  the threshold is crossed.
- Ring: 44-unit SVG, r 20, rotated −90°; `strokeDashoffset = C × (1 −
progress)` via `useTransform`, track at `stroke-border/70`.
- Entrance: spring (480/32) from y 12, scale 0.9, opacity 0 inside
  `AnimatePresence`.
- Button: `size-11 rounded-full bg-card ring-hairline shadow-raised-md
inset-shadow-highlight`, `hover:shadow-raised-lg`, `active:scale-95`.

<!-- PEBBLE-LICENSE -->

registry/button/back-to-top/back-to-top.tsx

"use client";

/* PEBBLE-LICENSE
Built by Pebble. Component: back-to-top.

First-party license: MIT

The following first-party terms apply only to original Pebble material.

MIT License

Copyright (c) 2026 Praxys Solutions LLC (Pebble)

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
END PEBBLE-LICENSE */

import * as React from "react";
import {
  AnimatePresence,
  motion,
  MotionConfig,
  useMotionValue,
  useReducedMotion,
  useTransform,
} from "motion/react";

import { cn } from "@/lib/utils";

const R = 20;
const C = 2 * Math.PI * R;

export type BackToTopProps = Omit<React.ComponentProps<"button">, "children"> & {
  /**
   * The scroll container. Omit to follow the window (the button is then
   * `fixed`); with a container, put the button in a `relative` wrapper beside
   * it and it sits `absolute` over the container's corner.
   */
  target?: React.RefObject<HTMLElement | null>;
  /** Show once scrolled this far (px). */
  threshold?: number;
  position?: "bottom-right" | "bottom-left" | "bottom-center";
  /** Draw the scroll-position ring around the button. */
  showProgress?: boolean;
  label?: string;
};

/**
 * A floating button that appears once a page or container has been scrolled,
 * shows how far down you are as a ring, and scrolls smoothly back to the top
 * (instantly under reduced motion).
 */
export function BackToTop({
  target,
  threshold = 240,
  position = "bottom-right",
  showProgress = true,
  label = "Back to top",
  className,
  onClick,
  ...props
}: BackToTopProps) {
  const reduced = useReducedMotion();
  const [visible, setVisible] = React.useState(false);
  const progress = useMotionValue(0);
  const offset = useTransform(progress, (p) => C * (1 - p));

  React.useEffect(() => {
    const el = target?.current ?? null;
    const read = () => {
      const top = el ? el.scrollTop : window.scrollY;
      const room = el
        ? el.scrollHeight - el.clientHeight
        : document.documentElement.scrollHeight - window.innerHeight;
      progress.set(room > 0 ? Math.min(1, top / room) : 0);
      setVisible(top > threshold);
    };
    const source: HTMLElement | Window = el ?? window;
    source.addEventListener("scroll", read, { passive: true });
    window.addEventListener("resize", read);
    const frame = requestAnimationFrame(read);
    return () => {
      cancelAnimationFrame(frame);
      source.removeEventListener("scroll", read);
      window.removeEventListener("resize", read);
    };
  }, [target, threshold, progress]);

  function scrollUp(event: React.MouseEvent<HTMLButtonElement>) {
    onClick?.(event);
    if (event.defaultPrevented) return;
    const behavior: ScrollBehavior = reduced ? "auto" : "smooth";
    const el = target?.current;
    if (el) el.scrollTo({ top: 0, behavior });
    else window.scrollTo({ top: 0, behavior });
  }

  return (
    <MotionConfig reducedMotion="user">
      <AnimatePresence>
        {visible && (
          <motion.div
            key="back-to-top"
            initial={{ opacity: 0, y: 12, scale: 0.9 }}
            animate={{ opacity: 1, y: 0, scale: 1 }}
            exit={{ opacity: 0, y: 12, scale: 0.9 }}
            transition={{ type: "spring", stiffness: 480, damping: 32 }}
            className={cn(
              "z-40",
              target ? "absolute" : "fixed",
              position === "bottom-right" && "right-4 bottom-4",
              position === "bottom-left" && "bottom-4 left-4",
              position === "bottom-center" && "inset-x-0 bottom-4 mx-auto w-fit",
            )}
          >
            <button
              type="button"
              data-slot="back-to-top"
              aria-label={label}
              title={label}
              onClick={scrollUp}
              className={cn(
                "group/top relative grid size-11 cursor-pointer place-items-center rounded-full bg-card text-foreground",
                "shadow-raised-md ring-1 inset-shadow-highlight ring-hairline",
                "transition-[box-shadow,transform] duration-(--motion-fast) ease-(--motion-move) hover:shadow-raised-lg active:scale-95",
                "focus-visible:ring-2 focus-visible:ring-ring focus-visible:outline-none motion-reduce:active:scale-100",
                className,
              )}
              {...props}
            >
              {showProgress && (
                <svg viewBox="0 0 44 44" aria-hidden="true" className="absolute inset-0 -rotate-90">
                  <circle
                    cx="22"
                    cy="22"
                    r={R}
                    fill="none"
                    strokeWidth="2"
                    className="stroke-border/70"
                  />
                  <motion.circle
                    cx="22"
                    cy="22"
                    r={R}
                    fill="none"
                    strokeWidth="2"
                    strokeLinecap="round"
                    strokeDasharray={C}
                    style={{ strokeDashoffset: offset }}
                    className="stroke-accent"
                  />
                </svg>
              )}
              <svg
                viewBox="0 0 16 16"
                fill="none"
                stroke="currentColor"
                strokeWidth="1.75"
                strokeLinecap="round"
                strokeLinejoin="round"
                aria-hidden="true"
                className="relative size-4 transition-transform duration-(--motion-fast) group-hover/top:-translate-y-0.5 motion-reduce:transition-none"
              >
                <path d="M8 13V3.5M3.5 8 8 3.5 12.5 8" />
              </svg>
            </button>
          </motion.div>
        )}
      </AnimatePresence>
    </MotionConfig>
  );
}