A black circle follows the mouse pointer across a blank SVG canvas. The example demonstrates interactive state management in React, where mouse coordinates update the circle’s position in real time. It uses React’s `useState` and `useCallback` hooks to track and handle mouse movement events, while d3.v5 provides the SVG rendering utilities. The circle’s center is set directly from the `clientX` and `clientY` properties of the browser’s mouse event, with no additional data or animation logic.