feat(web-ui): SplitPane layout component — draggable, collapsible, persistent (#507)

- Draggable divider updates pane widths in real-time via direct DOM
  mutation during mousemove (no setState jank), flush to state on mouseup
- Collapse/expand buttons on each pane edge (ArrowLeft01Icon/ArrowRight01Icon)
- Split position persists to localStorage; restored on mount
- minPanePercent enforced (default 15%) during drag
- Smooth CSS transition on collapse/expand, disabled during drag
- Mobile (<768px): vertical stack layout, divider hidden
- 15 tests covering all acceptance criteria
- Adds ArrowLeft01Icon to @hugeicons/react mock

Co-authored-by: Test User <test@example.com>

Author: frankbria

web-ui/__mocks__/@hugeicons/react.js

@@ -58,4 +58,6 @@ module.exports = {
   // AgentChatPanel
   ArrowRight01Icon: createIconMock('ArrowRight01Icon'),
   Alert01Icon: createIconMock('Alert01Icon'),
+  // SplitPane
+  ArrowLeft01Icon: createIconMock('ArrowLeft01Icon'),
 };

web-ui/src/__tests__/components/sessions/SplitPane.test.tsx

@@ -0,0 +1,201 @@
+import { render, screen, fireEvent, act } from '@testing-library/react';
+import { SplitPane } from '@/components/sessions/SplitPane';
+
+// ── localStorage mock ────────────────────────────────────────────────────
+
+const localStorageMock = (() => {
+  let store: Record<string, string> = {};
+  return {
+    getItem: (key: string) => store[key] ?? null,
+    setItem: (key: string, value: string) => {
+      store[key] = value;
+    },
+    removeItem: (key: string) => {
+      delete store[key];
+    },
+    clear: () => {
+      store = {};
+    },
+  };
+})();
+
+Object.defineProperty(window, 'localStorage', { value: localStorageMock });
+
+// ── matchMedia mock ──────────────────────────────────────────────────────
+
+function mockMatchMedia(matches: boolean) {
+  Object.defineProperty(window, 'matchMedia', {
+    writable: true,
+    value: jest.fn().mockImplementation((query: string) => ({
+      matches,
+      media: query,
+      onchange: null,
+      addListener: jest.fn(),
+      removeListener: jest.fn(),
+      addEventListener: jest.fn(),
+      removeEventListener: jest.fn(),
+      dispatchEvent: jest.fn(),
+    })),
+  });
+}
+
+// ── getBoundingClientRect mock ───────────────────────────────────────────
+
+function mockContainerRect(left = 0, width = 1000) {
+  jest.spyOn(Element.prototype, 'getBoundingClientRect').mockReturnValue({
+    left,
+    width,
+    top: 0,
+    right: left + width,
+    bottom: 100,
+    height: 100,
+    x: left,
+    y: 0,
+    toJSON: () => ({}),
+  });
+}
+
+// ── Tests ────────────────────────────────────────────────────────────────
+
+beforeEach(() => {
+  localStorageMock.clear();
+  mockMatchMedia(true); // desktop by default
+});
+
+afterEach(() => {
+  jest.restoreAllMocks();
+});
+
+describe('SplitPane', () => {
+  it('renders left and right children', () => {
+    render(<SplitPane left={<div>Left content</div>} right={<div>Right content</div>} />);
+    expect(screen.getByText('Left content')).toBeInTheDocument();
+    expect(screen.getByText('Right content')).toBeInTheDocument();
+  });
+
+  it('uses defaultSplit=45 when no localStorage value exists', () => {
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} defaultSplit={45} />);
+    const leftPane = screen.getByTestId('split-pane-left');
+    expect(leftPane).toHaveStyle({ width: '45%' });
+  });
+
+  it('restores split position from localStorage on mount', () => {
+    localStorageMock.setItem('split-pane-position', '60');
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} />);
+    const leftPane = screen.getByTestId('split-pane-left');
+    expect(leftPane).toHaveStyle({ width: '60%' });
+  });
+
+  it('uses custom storageKey for localStorage', () => {
+    localStorageMock.setItem('my-custom-key', '70');
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} storageKey="my-custom-key" />);
+    const leftPane = screen.getByTestId('split-pane-left');
+    expect(leftPane).toHaveStyle({ width: '70%' });
+  });
+
+  it('persists position to localStorage on drag end', () => {
+    mockContainerRect(0, 1000);
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} storageKey="test-key" />);
+    const divider = screen.getByTestId('split-pane-divider');
+
+    fireEvent.mouseDown(divider);
+    fireEvent.mouseMove(document, { clientX: 600 });
+    fireEvent.mouseUp(document);
+
+    expect(localStorageMock.getItem('test-key')).toBe('60');
+  });
+
+  it('enforces minPanePercent during drag (default 15%)', () => {
+    mockContainerRect(0, 1000);
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} storageKey="test-key" />);
+    const divider = screen.getByTestId('split-pane-divider');
+
+    fireEvent.mouseDown(divider);
+    fireEvent.mouseMove(document, { clientX: 50 }); // 5% — below min
+    fireEvent.mouseUp(document);
+
+    // Should be clamped to 15%
+    expect(localStorageMock.getItem('test-key')).toBe('15');
+  });
+
+  it('enforces minPanePercent on the right side during drag', () => {
+    mockContainerRect(0, 1000);
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} storageKey="test-key" />);
+    const divider = screen.getByTestId('split-pane-divider');
+
+    fireEvent.mouseDown(divider);
+    fireEvent.mouseMove(document, { clientX: 980 }); // 98% — right pane below min
+    fireEvent.mouseUp(document);
+
+    // Should be clamped to 85% (100 - 15)
+    expect(localStorageMock.getItem('test-key')).toBe('85');
+  });
+
+  it('collapses left pane when left collapse button is clicked', () => {
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} />);
+    const collapseLeft = screen.getByTestId('collapse-left');
+    fireEvent.click(collapseLeft);
+    const leftPane = screen.getByTestId('split-pane-left');
+    expect(leftPane).toHaveStyle({ width: '0%' });
+  });
+
+  it('expands left pane when collapse button is clicked again', () => {
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} defaultSplit={45} />);
+    const collapseLeft = screen.getByTestId('collapse-left');
+    fireEvent.click(collapseLeft); // collapse
+    fireEvent.click(collapseLeft); // expand
+    const leftPane = screen.getByTestId('split-pane-left');
+    expect(leftPane).toHaveStyle({ width: '45%' });
+  });
+
+  it('collapses right pane when right collapse button is clicked', () => {
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} />);
+    const collapseRight = screen.getByTestId('collapse-right');
+    fireEvent.click(collapseRight);
+    const rightPane = screen.getByTestId('split-pane-right');
+    expect(rightPane).toHaveStyle({ width: '0%' });
+  });
+
+  it('expands right pane when collapse button is clicked again', () => {
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} defaultSplit={45} />);
+    const collapseRight = screen.getByTestId('collapse-right');
+    fireEvent.click(collapseRight); // collapse
+    fireEvent.click(collapseRight); // expand
+    const rightPane = screen.getByTestId('split-pane-right');
+    expect(rightPane).toHaveStyle({ width: '55%' });
+  });
+
+  it('does not apply inline width styles on mobile', () => {
+    mockMatchMedia(false); // mobile
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} />);
+    const container = screen.getByTestId('split-pane-container');
+    expect(container.className).toContain('flex-col');
+  });
+
+  it('hides divider on mobile', () => {
+    mockMatchMedia(false);
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} />);
+    const divider = screen.getByTestId('split-pane-divider');
+    expect(divider.className).toContain('hidden');
+  });
+
+  it('applies custom className to outer container', () => {
+    render(
+      <SplitPane left={<div>L</div>} right={<div>R</div>} className="my-custom-class" />,
+    );
+    const container = screen.getByTestId('split-pane-container');
+    expect(container.className).toContain('my-custom-class');
+  });
+
+  it('does not move divider if mouse was not pressed (no drag started)', () => {
+    localStorageMock.setItem('split-pane-position', '45');
+    mockContainerRect(0, 1000);
+    render(<SplitPane left={<div>L</div>} right={<div>R</div>} storageKey="split-pane-position" />);
+
+    // Move without mousedown
+    fireEvent.mouseMove(document, { clientX: 700 });
+    fireEvent.mouseUp(document);
+
+    expect(localStorageMock.getItem('split-pane-position')).toBe('45');
+  });
+});