feat(ui): Add a Mosaic scroll area with scroll-driven edge fades

.changeset/gentle-hoops-relax.md

@@ -0,0 +1,2 @@
+---
+---

packages/swingset/CLAUDE.md

@@ -57,7 +57,7 @@ Pick the archetype below by the component's **layer** (its `meta.group`), then f
 
 ### Layers
 
-`meta.group` places a component in one of six layers. Sidebar order follows the `registry` array; group order follows first appearance there. Use these exact group strings:
+`meta.group` places an entry in one of these layers. Sidebar order follows the `registry` array; group order follows first appearance there. Use these exact group strings:
 
 | Group        | What lives here                                                | Archetype |
 | ------------ | -------------------------------------------------------------- | --------- |
@@ -67,9 +67,18 @@ Pick the archetype below by the component's **layer** (its `meta.group`), then f
 | `Blocks`     | Reusable composite UI (e.g. `Destructive`)                     | C         |
 | `Components` | Styled Mosaic components — simple CVA recipe (`Button`, `Input`) or compound/slot-based (`Dialog`, `Tabs`) | A         |
 | `Primitives` | Headless `@clerk/headless` primitives (`Accordion`)            | B         |
+| `Styles`     | Atomic styles that ship as StyleX atoms, not components (`Scroll Area`) | B (adapted) |
+| `Hooks`      | Headless hooks (`useDataTable`)                                | B (adapted) |
 
 `AIO` → `Panels` → `Sections` → `Blocks` → `Components` → `Primitives` runs roughly high-level-composition → low-level-primitive. Composed layers (AIO/Panels/Sections/Blocks) are documented as compositions of lower layers (archetype C); leaf layers (Components, Primitives) get full prop/knob docs (archetypes A and B).
 
+`Styles` and `Hooks` are the non-component layers: there is no element to knob, so they follow
+archetype B's shape (Example → Usage → Parts → Styling) with `Props` replaced by whatever the export
+actually surfaces — an argument table for a style function, a return-value table for a hook. A
+`Styles` entry documents the theme tokens its atoms read, since those tokens _are_ its API; the
+`Hooks` entry (`use-data-table.stories.tsx`) is `meta` alone, with no story exports at all, which is
+the minimum a section entry needs.
+
 Archetype A has two forms, chosen by whether the component exposes a single flat CVA recipe: **simple** components (`Button`, `Input`) are knob-driven; **compound** components built from slot recipes (`Dialog`, `Tabs`) have no flat variant props to knob, so they're documented like a primitive but themed. Both are detailed under Archetype A below.
 
 ### `meta` conventions (all archetypes)

packages/swingset/src/components/ClientRoot.tsx

@@ -13,17 +13,25 @@ import {
 } from '@/components/ui/breadcrumb';
 import { Separator } from '@/components/ui/separator';
 import { SidebarInset, SidebarProvider, SidebarTrigger } from '@/components/ui/sidebar';
+import { getModule } from '@/lib/registry';
 
 import { AppSidebar } from './app-sidebar';
 import { ThemeToggle } from './ThemeToggle';
 
 function useBreadcrumb() {
   const pathname = usePathname();
   // /components/button → ["Button"]
-  // /primitives/dialog → ["Dialog"]
-  // The first segment is the group; drop it and surface the component (plus any sub-path).
-  const parts = pathname.split('/').filter(Boolean).slice(1);
-  return parts.map(p => p.charAt(0).toUpperCase() + p.slice(1).replace(/-/g, ' '));
+  // /styles/scroll-area → ["Scroll Area"]
+  // The first segment is the group; drop it and surface the entry (plus any sub-path).
+  const [groupSlug, ...parts] = pathname.split('/').filter(Boolean);
+
+  // Prefer the registry's own `meta.title`, which is the only source that round-trips a slug back
+  // to how the entry is actually written — `scroll-area` → `Scroll Area`, `use-data-table` →
+  // `useDataTable`. Fall back to title-casing the slug for any path the registry doesn't cover.
+  return parts.map((part, index) => {
+    const title = index === 0 ? getModule(groupSlug, part)?.meta.title : undefined;
+    return title ?? part.replace(/(^|-)([a-z])/g, (_, sep: string, ch: string) => (sep ? ' ' : '') + ch.toUpperCase());
+  });
 }
 
 export function ClientRoot({ children }: { children: React.ReactNode }) {

packages/swingset/src/components/DocsViewer.tsx

@@ -57,6 +57,10 @@ const docModules: Record<string, Record<string, React.ComponentType>> = {
     tabs: dynamic(() => import('../stories/tabs.mdx')),
     tooltip: dynamic(() => import('../stories/tooltip.mdx')),
   },
+  styles: {
+    // Atomic styles — shipped as StyleX atoms rather than components.
+    'scroll-area': dynamic(() => import('../stories/scroll-area.mdx')),
+  },
   hooks: {
     // Headless hooks — alphabetical.
     'use-data-table': dynamic(() => import('../stories/use-data-table.mdx')),

packages/swingset/src/components/app-sidebar.tsx

@@ -71,10 +71,16 @@ export function AppSidebar({ ...props }: React.ComponentProps<typeof Sidebar>) {
               <SidebarMenu>
                 {components.map(({ mod, componentSlug }) => {
                   const href = `/${groupSlug}/${componentSlug}`;
-                  // Hooks (e.g. `useDataTable`) are called, not rendered — show `useX()` rather
-                  // than JSX `<useX />`. Everything else is a component.
-                  const isHook = /^use[A-Z]/.test(mod.meta.title);
-                  const usage = isHook ? `${mod.meta.title}()` : `<${mod.meta.title} />`;
+                  // How an entry is USED differs by layer, so the label follows the layer rather
+                  // than a guess at the title: hooks are called, atomic styles are a set of
+                  // exports with no single call form worth privileging, and everything else is a
+                  // component rendered as JSX.
+                  const usage =
+                    mod.meta.group === 'Hooks'
+                      ? `${mod.meta.title}()`
+                      : mod.meta.group === 'Styles'
+                        ? mod.meta.title
+                        : `<${mod.meta.title} />`;
                   return (
                     <SidebarMenuItem key={mod.meta.title}>
                       <SidebarMenuButton

packages/swingset/src/lib/registry.ts

@@ -51,6 +51,7 @@ import {
   Group as ItemGroup,
   Interactive as ItemInteractive,
   meta as itemMeta,
+  Scrolling as ItemScrolling,
 } from '../stories/item.stories';
 import { Default as MenuComponentDefault, meta as menuComponentMeta } from '../stories/menu.component.stories';
 import { meta as menuMeta } from '../stories/menu.stories';
@@ -94,6 +95,14 @@ import {
   Placement as PopoverComponentPlacement,
 } from '../stories/popover.component.stories';
 import { meta as popoverMeta } from '../stories/popover.stories';
+import {
+  Default as ScrollAreaDefault,
+  Gutter as ScrollAreaGutter,
+  HoverReveal as ScrollAreaHoverReveal,
+  meta as scrollAreaMeta,
+  NotScrollable as ScrollAreaNotScrollable,
+  ThemedScrollbar as ScrollAreaThemedScrollbar,
+} from '../stories/scroll-area.stories';
 import { meta as selectMeta } from '../stories/select.stories';
 import { Default as TabsComponentDefault, meta as tabsComponentMeta } from '../stories/tabs.component.stories';
 import { meta as tabsMeta } from '../stories/tabs.stories';
@@ -174,6 +183,7 @@ const itemModule: StoryModule = {
   Default: ItemDefault,
   Interactive: ItemInteractive,
   Group: ItemGroup,
+  Scrolling: ItemScrolling,
 };
 
 const headingModule: StoryModule = {
@@ -213,6 +223,15 @@ const selectModule: StoryModule = { meta: selectMeta };
 const tabsModule: StoryModule = { meta: tabsMeta };
 const tooltipModule: StoryModule = { meta: tooltipMeta };
 
+const scrollAreaModule: StoryModule = {
+  meta: scrollAreaMeta,
+  Default: ScrollAreaDefault,
+  NotScrollable: ScrollAreaNotScrollable,
+  Gutter: ScrollAreaGutter,
+  HoverReveal: ScrollAreaHoverReveal,
+  ThemedScrollbar: ScrollAreaThemedScrollbar,
+};
+
 const useDataTableModule: StoryModule = { meta: useDataTableMeta };
 
 export const registry: StoryModule[] = [
@@ -254,6 +273,8 @@ export const registry: StoryModule[] = [
   selectModule,
   tabsModule,
   tooltipModule,
+  // Styles — atomic styles that ship as StyleX atoms rather than components.
+  scrollAreaModule,
   // Hooks — alphabetical within the group.
   useDataTableModule,
 ];

packages/swingset/src/stories/item.mdx

@@ -34,6 +34,33 @@ Set `size` once on `Item.Root` and the row scales as a unit: it fixes the row's
   storyModule={ItemStories}
 />
 
+### Scrolling
+
+`Item.Group` is the canonical scroll surface in Mosaic: cap its height, spread the scroll-area atoms
+onto it, and it fades its content at whichever edge still has something to reveal.
+
+<Story
+  name='Scrolling'
+  storyModule={ItemStories}
+/>
+
+```tsx
+import { scrollAreaRoot, scrollAreaViewport } from '@clerk/ui/mosaic/components/scroll-area';
+import * as stylex from '@stylexjs/stylex';
+
+<div
+  {...stylex.props(scrollAreaRoot)}
+  style={{ height: 260 }}
+>
+  <Item.Group {...stylex.props(...scrollAreaViewport())}>{organizations}</Item.Group>
+</div>;
+```
+
+The atoms aren't specific to `Item` — they go on anything that scrolls, and they carry the edge
+fades, the scrollbar, the gutter, and the theming tokens with them. See
+[Scroll Area](/styles/scroll-area) for the full surface: the gutter argument, what happens when
+there is nothing to scroll, the token table, and how to replace the fade entirely.
+
 ## Usage
 
 ```tsx

packages/swingset/src/stories/item.stories.tsx

@@ -2,6 +2,9 @@
 import { Avatar } from '@clerk/ui/mosaic/components/avatar';
 import { Button } from '@clerk/ui/mosaic/components/button';
 import { Item } from '@clerk/ui/mosaic/components/item';
+import { scrollAreaRoot, scrollAreaViewport } from '@clerk/ui/mosaic/components/scroll-area';
+import { radiusVars } from '@clerk/ui/mosaic/styles';
+import * as stylex from '@stylexjs/stylex';
 import * as React from 'react';
 
 import type { StoryMeta } from '@/lib/types';
@@ -332,3 +335,68 @@ export function Group() {
     </div>
   );
 }
+
+const organizations = [
+  'Clerk',
+  'Acme Corporation',
+  'Globex',
+  'Initech',
+  'Umbrella Health',
+  'DesignCloud',
+  'Stark Industries',
+  'Wayne Enterprises',
+  'Cyberdyne Systems',
+  'Soylent Industries',
+  'Tyrell Corporation',
+  'Weyland-Yutani',
+];
+
+// `Item.Group` is the canonical scroll surface, so this shows the atoms doing the minimum: cap a
+// height, spread them on, and the group fades its own edges. The Scroll Area page under Styles
+// carries the full surface — the gutter argument, the resting state, and the theming tokens.
+export function Scrolling() {
+  // `stylex.props()` returns a `className`, so it has to be MERGED with any class of your own
+  // rather than spread beside one — whichever comes last in JSX wins outright.
+  const root = stylex.props(scrollAreaRoot);
+
+  return (
+    <div
+      {...root}
+      className={`${root.className} border-border w-full border`}
+      style={{ height: 200, borderRadius: radiusVars['--cl-radius-inner'] }}
+    >
+      <Item.Group {...stylex.props(...scrollAreaViewport())}>
+        {organizations.map(name => (
+          <Item.Root
+            key={name}
+            size='xs'
+            render={({ children, ...props }) => (
+              <button
+                {...props}
+                type='button'
+              >
+                {children}
+              </button>
+            )}
+          >
+            <Item.Media>
+              <Avatar.Root
+                size='fit'
+                shape='square'
+              >
+                <Avatar.Image
+                  src='https://github.com/clerk.png'
+                  alt={name}
+                />
+                <Avatar.Fallback>{name[0]}</Avatar.Fallback>
+              </Avatar.Root>
+            </Item.Media>
+            <Item.Content>
+              <Item.Title>{name}</Item.Title>
+            </Item.Content>
+          </Item.Root>
+        ))}
+      </Item.Group>
+    </div>
+  );
+}