A React application renders a single black circle that follows the mouse cursor across the full browser window. The circle repositions dynamically based on the user's pointer movements, updating its coordinates in real time. The implementation uses React's `useState` and `useCallback` hooks to track the mouse position and optimize the event handler, while ReactDOM renders the SVG element with an `onMouseMove` listener. The SVG viewport fills the entire viewport, with the circle radius fixed at 30 pixels.