test: add tests

Author: bydawen

src/components/NavigationBar/tabs/useIndexOfLastVisibleChild.test.jsx

@@ -0,0 +1,36 @@
+import React from 'react';
+
+import { render } from '@testing-library/react';
+
+import useIndexOfLastVisibleChild from './useIndexOfLastVisibleChild';
+
+describe('useIndexOfLastVisibleChild', () => {
+  let observeMock;
+  let disconnectMock;
+
+  beforeAll(() => {
+    observeMock = jest.fn();
+    disconnectMock = jest.fn();
+    global.ResizeObserver = class {
+      observe = observeMock;
+
+      disconnect = disconnectMock;
+    };
+  });
+
+  afterAll(() => {
+    delete global.ResizeObserver;
+  });
+
+  it('runs effect and cleanup (covers L52–53, L66)', () => {
+    const TestComponent = () => {
+      const [, containerRef] = useIndexOfLastVisibleChild();
+      return <div ref={containerRef} />;
+    };
+
+    const { unmount } = render(<TestComponent />);
+    unmount();
+
+    expect(disconnectMock).toHaveBeenCalled();
+  });
+});