A React SVG circle follows the mouse cursor across the full browser window, updating its position in real time. The app uses React’s `useState` and `useCallback` hooks to track the cursor’s `clientX` and `clientY` coordinates, storing them in a state object. The `onMouseMove` event handler on the `<svg>` element triggers a state update, and the circle’s `cx` and `cy` attributes re-render with the new position. The example demonstrates declarative interaction between React state and SVG rendering without external data or animation libraries.