Scroll Area

Built on the Radix ScrollArea primitive. The Viewport is a native overflow
container whose own scrollbars Radix hides with injected CSS; Radix measures
viewport and content sizes (ResizeObserver) and renders absolutely positioned
`Scrollbar` tracks with a `Thumb` sized to the visible ratio and translated to
the scroll offset. Dragging the thumb or clicking the track scrolls the
viewport. The root is `relative overflow-hidden`; the viewport is
`size-full rounded-[inherit]` so it fills the box and inherits its corners.
Bars are 10 px (`w-2.5`/`h-2.5`) with 1 px padding and a transparent border
on the inner edge, so the thumb sits off the content. The thumb is
`bg-muted-foreground/40` (60% on hover) so it reads on both card and page
backgrounds, `CanvasText` in forced colors. Radix mounts the thumb only after
it has measured overflow. Differences from the source: a `scrollbars` prop
instead of nesting a horizontal `ScrollBar` inside the content, the viewport is
focusable and labelable, an opaque `ring` focus style, and a stronger thumb
than the `border` colour.

**Pebble (2026-11-03).** The bar is a `group/scrollbar` rail with `p-[3px]`
that widens on hover (`w-2.5 → w-3.5`, `h-2.5 → h-3.5`) and takes a
`bg-muted/70` track, with a motion-safe `duration-(--motion-fast) ease-enter`
transition. The thumb is `bg-muted-foreground/35`, `/60` on bar hover and
`/75` while dragged, and its `before:` pseudo-element is at least 32 px along
the bar's long axis (`min-h-8` vertical, `min-w-8` horizontal) so it is easy
to grab without widening the bar across. Focus is an inset 2 px `ring`-colour
outline on the root's `::after`, outside the viewport's mask and stacked over
the bars. New
optional `fade`: an effect measures the viewport on scroll and with a
ResizeObserver, and applies an inline `mask-image` linear gradient that
fades 1.5 rem at whichever edge has more to scroll. It starts with no fade so
the server render matches, and as an alpha mask it works on any surface
colour. The demo shows a chat channel with a vertical fade and unread ember
dots, and a horizontal file strip in a recessed well with drawn SVG
thumbnails.

<!-- PEBBLE-LICENSE -->

registry/layout/scroll-area/scroll-area.tsx

"use client";

/* PEBBLE-LICENSE
Built by Pebble. Component: scroll-area.

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.

THIRD-PARTY NOTICES
These notices apply to identified upstream portions; Pebble terms do not replace or narrow their rights.

Upstream: https://raw.githubusercontent.com/shadcn-ui/ui/main/LICENSE.md
MIT License

Copyright (c) 2023 shadcn

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 { ScrollArea as ScrollAreaPrimitive } from "radix-ui";

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

export type ScrollAreaProps = React.ComponentProps<typeof ScrollAreaPrimitive.Root> & {
  /** Which custom scrollbars to render. */
  scrollbars?: "vertical" | "horizontal" | "both";
  /**
   * Fade the content out at an edge that has more to scroll, so the overflow
   * is visible before the user scrolls. Follows the vertical axis, or the
   * horizontal one when `scrollbars="horizontal"`.
   */
  fade?: boolean;
};

const FADE = "1.5rem";

export function ScrollArea({
  scrollbars = "vertical",
  fade = false,
  className,
  children,
  "aria-label": ariaLabel,
  "aria-labelledby": ariaLabelledBy,
  ...props
}: ScrollAreaProps) {
  const labelled = ariaLabel !== undefined || ariaLabelledBy !== undefined;
  const viewport = React.useRef<HTMLDivElement>(null);
  const horizontal = scrollbars === "horizontal";
  // Which edges have more content beyond them. Starts with none, so the
  // server render and first paint are the plain list; measured after mount.
  const [edges, setEdges] = React.useState({ start: false, end: false });

  React.useEffect(() => {
    const el = viewport.current;
    if (!fade || !el) return;
    const measure = () => {
      const pos = horizontal ? el.scrollLeft : el.scrollTop;
      const max = horizontal ? el.scrollWidth - el.clientWidth : el.scrollHeight - el.clientHeight;
      const start = pos > 1;
      const end = pos < max - 1;
      setEdges((prev) => (prev.start === start && prev.end === end ? prev : { start, end }));
    };
    measure();
    el.addEventListener("scroll", measure, { passive: true });
    const ro = typeof ResizeObserver === "undefined" ? undefined : new ResizeObserver(measure);
    ro?.observe(el);
    if (el.firstElementChild) ro?.observe(el.firstElementChild);
    return () => {
      el.removeEventListener("scroll", measure);
      ro?.disconnect();
    };
  }, [fade, horizontal]);

  const mask = fade
    ? `linear-gradient(to ${horizontal ? "right" : "bottom"}, transparent 0, black ${
        edges.start ? FADE : "0px"
      }, black calc(100% - ${edges.end ? FADE : "0px"}), transparent 100%)`
    : undefined;

  return (
    <ScrollAreaPrimitive.Root
      data-slot="scroll-area"
      className={cn(
        "relative overflow-hidden",
        // The focus ring lives on the root, outside the viewport's fade mask,
        // drawn by a pseudo-element stacked over the z-10 scrollbars so a
        // hovered bar's fill cannot cover it. An outline survives forced
        // colors. Only the own viewport counts, not a focused child or a
        // nested scroll area.
        "after:pointer-events-none after:absolute after:inset-0 after:z-20 after:rounded-[inherit]",
        "has-[>[data-slot=scroll-area-viewport]:focus-visible]:after:outline-2 has-[>[data-slot=scroll-area-viewport]:focus-visible]:after:-outline-offset-2 has-[>[data-slot=scroll-area-viewport]:focus-visible]:after:outline-ring",
        className,
      )}
      {...props}
    >
      <ScrollAreaPrimitive.Viewport
        ref={viewport}
        data-slot="scroll-area-viewport"
        data-fade-start={fade ? edges.start : undefined}
        data-fade-end={fade ? edges.end : undefined}
        // Focusable so keyboard users can scroll it with arrow keys and Page
        // Up/Down. A label turns it into a named region landmark.
        tabIndex={0}
        role={labelled ? "region" : undefined}
        aria-label={ariaLabel}
        aria-labelledby={ariaLabelledBy}
        // The fade is an alpha mask, not a colour, so it works on any surface.
        style={mask ? { maskImage: mask, WebkitMaskImage: mask } : undefined}
        className="size-full rounded-[inherit] outline-none"
      >
        {children}
      </ScrollAreaPrimitive.Viewport>
      {scrollbars !== "horizontal" && <ScrollBar orientation="vertical" />}
      {scrollbars !== "vertical" && <ScrollBar orientation="horizontal" />}
      {scrollbars === "both" && <ScrollAreaPrimitive.Corner data-slot="scroll-area-corner" />}
    </ScrollAreaPrimitive.Root>
  );
}

export function ScrollBar({
  orientation = "vertical",
  className,
  ...props
}: React.ComponentProps<typeof ScrollAreaPrimitive.Scrollbar>) {
  return (
    <ScrollAreaPrimitive.Scrollbar
      data-slot="scroll-area-scrollbar"
      orientation={orientation}
      className={cn(
        // A slim rail that widens and takes a recessed track on hover or drag,
        // so it is easy to grab without crowding the content at rest.
        "group/scrollbar z-10 flex touch-none p-[3px] select-none",
        "hover:bg-muted/70 active:bg-muted/70",
        "motion-safe:transition-[background-color,width,height] motion-safe:duration-(--motion-fast) motion-safe:ease-enter",
        orientation === "vertical" && "h-full w-2.5 hover:w-3.5",
        orientation === "horizontal" && "h-2.5 flex-col hover:h-3.5",
        className,
      )}
      {...props}
    >
      <ScrollAreaPrimitive.Thumb
        data-slot="scroll-area-thumb"
        className={cn(
          "relative flex-1 rounded-full bg-muted-foreground/35",
          "group-hover/scrollbar:bg-muted-foreground/60 active:bg-muted-foreground/75",
          "motion-safe:transition-colors motion-safe:duration-(--motion-fast)",
          // Grow the hit area beyond the visible thumb for touch and mice, along
          // the bar only, so it never reaches across into the content.
          "before:absolute before:top-1/2 before:left-1/2 before:size-full before:-translate-1/2 before:content-['']",
          "group-data-[orientation=horizontal]/scrollbar:before:min-w-8 group-data-[orientation=vertical]/scrollbar:before:min-h-8",
          "forced-colors:bg-[CanvasText]",
        )}
      />
    </ScrollAreaPrimitive.Scrollbar>
  );
}