A single circle tracks the mouse cursor in real time across a full-screen SVG canvas. Built with React 18, the app uses the `useState` hook to store cursor coordinates from the `onMouseMove` event on a div catcher, then passes those coordinates to an SVG circle’s `cx` and `cy` attributes for immediate repositioning. The `useCallback` hook memoizes the event handler for efficiency. ReactDOM renders the component into the page’s root element.