A React component renders a single SVG circle that follows the mouse cursor across the viewport. The application tracks pointer movement through the SVG's `onMouseMove` event handler, updating React state with the cursor's `clientX` and `clientY` coordinates. D3 v5 is loaded via CDN but is not actively used in the interaction logic. The circle's `cx` and `cy` attributes bind directly to the state, causing the shape to re-render at the new position on every mouse movement.