A full-screen SVG tracks the mouse and draws a single black circle at the cursor's current position. The circle follows pointer movement in real time, rendered with React's `useState` and `useCallback` hooks to update its `cx` and `cy` coordinates on every `mousemove` event. The SVG uses `window.innerWidth` and `window.innerHeight` for its dimensions, and the example demonstrates declarative state-driven rendering with React 16.9 and ReactDOM.