A single black circle follows the user’s mouse movement across an SVG canvas. The circle’s center position updates in real time, tracking the cursor’s clientX and clientY coordinates. The example uses React’s `useState` and `useCallback` hooks to manage and update the mouse position, while D3’s `arc` is imported but not used in the visible code. The SVG renders within a React component, with a yellow static smiley face shown only as a commented-out reference in the HTML.