A full-screen SVG circle follows the mouse cursor, demonstrating how React state tracks changing pointer coordinates in real time. The example uses React’s `useState` and `useCallback` hooks to update the circle’s `cx` and `cy` attributes on every `mousemove` event. Rendered through ReactDOM, the SVG’s width and height match the browser window dimensions, while the circle’s radius is fixed at 30 pixels. No external data loading or D3 utilities appear in the visible code, despite the title.