A React mouse follower demonstrates a circle tracking the cursor across an SVG canvas. Using React's `useState` and `useCallback` hooks, the component updates the circle's `cx` and `cy` coordinates on every `onMouseOver` event, binding `clientX` and `clientY` to the SVG circle. The fixed 960×500 viewport starts the circle centered, and the lightweight interaction shows declarative state-driven rendering with React 17 and D3 v6 in the SVG namespace.