feat: add SplitPane composite for two-column list/detail layouts

Two-column grid layout with configurable ratio (1:1, 1:2, 2:3), list/detail
slots, and empty state message. Uses CSS custom property for dynamic grid
columns.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/composites/SplitPane/SplitPane.module.css

@@ -0,0 +1,37 @@
+.splitPane {
+  display: grid;
+  grid-template-columns: var(--split-columns, 1fr 2fr);
+  gap: 1px;
+  background: var(--border-subtle);
+  border: 1px solid var(--border-subtle);
+  border-radius: var(--radius-lg);
+  min-height: 0;
+  height: 100%;
+  box-shadow: var(--shadow-card);
+}
+
+.listPane {
+  background: var(--bg-surface);
+  display: flex;
+  flex-direction: column;
+  border-radius: var(--radius-lg) 0 0 var(--radius-lg);
+  overflow-y: auto;
+}
+
+.detailPane {
+  background: var(--bg-raised);
+  overflow-y: auto;
+  padding: 20px;
+  border-radius: 0 var(--radius-lg) var(--radius-lg) 0;
+}
+
+.emptyDetail {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  height: 100%;
+  color: var(--text-faint);
+  font-size: 13px;
+  font-family: var(--font-body);
+  font-style: italic;
+}

src/design-system/composites/SplitPane/SplitPane.test.tsx

@@ -0,0 +1,69 @@
+import { describe, it, expect } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import { SplitPane } from './SplitPane'
+
+describe('SplitPane', () => {
+  it('renders list and detail content', () => {
+    render(
+      <SplitPane
+        list={<div>List items</div>}
+        detail={<div>Detail content</div>}
+      />
+    )
+    expect(screen.getByText('List items')).toBeInTheDocument()
+    expect(screen.getByText('Detail content')).toBeInTheDocument()
+  })
+
+  it('shows default empty message when detail is null', () => {
+    render(
+      <SplitPane
+        list={<div>List items</div>}
+        detail={null}
+      />
+    )
+    expect(screen.getByText('Select an item to view details')).toBeInTheDocument()
+  })
+
+  it('shows custom empty message', () => {
+    render(
+      <SplitPane
+        list={<div>List items</div>}
+        detail={null}
+        emptyMessage="Pick something"
+      />
+    )
+    expect(screen.getByText('Pick something')).toBeInTheDocument()
+  })
+
+  it('renders with different ratios (checks --split-columns CSS property)', () => {
+    const { container, rerender } = render(
+      <SplitPane
+        list={<div>List</div>}
+        detail={<div>Detail</div>}
+        ratio="1:1"
+      />
+    )
+    const root = container.firstChild as HTMLElement
+    expect(root.style.getPropertyValue('--split-columns')).toBe('1fr 1fr')
+
+    rerender(
+      <SplitPane
+        list={<div>List</div>}
+        detail={<div>Detail</div>}
+        ratio="2:3"
+      />
+    )
+    expect(root.style.getPropertyValue('--split-columns')).toBe('2fr 3fr')
+  })
+
+  it('accepts className', () => {
+    const { container } = render(
+      <SplitPane
+        list={<div>List</div>}
+        detail={<div>Detail</div>}
+        className="custom-class"
+      />
+    )
+    expect(container.firstChild).toHaveClass('custom-class')
+  })
+})

src/design-system/composites/SplitPane/SplitPane.tsx

@@ -0,0 +1,38 @@
+import type { ReactNode } from 'react'
+import styles from './SplitPane.module.css'
+
+interface SplitPaneProps {
+  list: ReactNode
+  detail: ReactNode | null
+  emptyMessage?: string
+  ratio?: '1:1' | '1:2' | '2:3'
+  className?: string
+}
+
+const ratioMap: Record<string, string> = {
+  '1:1': '1fr 1fr',
+  '1:2': '1fr 2fr',
+  '2:3': '2fr 3fr',
+}
+
+export function SplitPane({
+  list,
+  detail,
+  emptyMessage = 'Select an item to view details',
+  ratio = '1:2',
+  className,
+}: SplitPaneProps) {
+  return (
+    <div
+      className={`${styles.splitPane} ${className ?? ''}`}
+      style={{ '--split-columns': ratioMap[ratio] } as React.CSSProperties}
+    >
+      <div className={styles.listPane}>{list}</div>
+      <div className={styles.detailPane}>
+        {detail !== null ? detail : (
+          <div className={styles.emptyDetail}>{emptyMessage}</div>
+        )}
+      </div>
+    </div>
+  )
+}

src/design-system/composites/index.ts

@@ -34,6 +34,7 @@ export { RouteFlow } from './RouteFlow/RouteFlow'
 export type { RouteNode } from './RouteFlow/RouteFlow'
 export { ShortcutsBar } from './ShortcutsBar/ShortcutsBar'
 export { SegmentedTabs } from './SegmentedTabs/SegmentedTabs'
+export { SplitPane } from './SplitPane/SplitPane'
 export { Tabs } from './Tabs/Tabs'
 export { ToastProvider, useToast } from './Toast/Toast'
 export { TreeView } from './TreeView/TreeView'