A black circle follows the mouse cursor across a full-screen SVG canvas. The visualization uses React with the `useState` and `useCallback` hooks to track the cursor's `clientX` and `clientY` coordinates on every `mousemove` event. The current mouse position is stored in state and then passed as the circle's `cx` and `cy` attributes, causing the shape to render at the cursor's location at all times. The code relies on ReactDOM to mount the component, while the initial position is set to the center of the window.