A single gray circle follows the user's mouse across a full-screen SVG canvas, updating its position in real time as the pointer moves. Built with React and ReactDOM, the application uses the `useState` and `useCallback` hooks to track the mouse's client coordinates via an SVG `onMouseMove` event handler. The circle's `cx` and `cy` attributes are bound to the tracked state, while the viewport dimensions are read directly from `window.innerWidth` and `window.innerHeight`.