This example shows a React application that renders a single SVG circle that follows the user's mouse cursor across the full browser viewport. The circle updates its position in real time using React's `useState` hook and an `onMouseMove` event handler that reads the cursor's client coordinates. The rendering relies on `ReactDOM.render` to mount the component to the HTML root element, while D3 v5 is loaded but not utilized in the visible interaction. The SVG viewport dimensions are set dynamically from `window.innerWidth` and `window.innerHeight`.