A black circle follows the mouse as it moves across a blank 960×500 SVG canvas. The example shows how React state updates, triggered by mouse events, can drive SVG attribute changes in real time. The React component `App` uses the `useState` hook to store the pointer’s `clientX` and `clientY` coordinates, updating them via a `handleMouseMove` callback on the `<svg>` element’s `onMouseMove` event. The circle’s `cx` and `cy` properties are bound directly to this state, rendering with d3.v7 and ReactDOM.