A circle follows the mouse cursor across an SVG canvas, with React managing its position in real time. The component uses React’s `useState` and `useCallback` hooks to track pointer coordinates from the `onMouseMove` event, updating the circle’s `cx` and `cy` attributes accordingly. The example demonstrates a minimal integration of D3.js (v5) with React, where D3 provides no visual functions—only the SVG rendering is handled by React, and the circle radius is fixed at 30 pixels. The canvas is 960 by 500 pixels, and the initial cursor position starts at the center.