A React component renders an SVG circle that follows the mouse cursor across a 960 by 500 viewport. The `App` component uses the `useState` and `useCallback` hooks to track and update the mouse position, binding an `onMouseMove` event handler to the SVG element. This example demonstrates integrating React's state management with D3 v5 and SVG rendering, showing how direct event handling replaces D3's typical selection and data-join pattern.