A circle follows the mouse cursor across a full-screen SVG canvas, updating its position in real time in response to pointer movement. The React app tracks the mouse coordinates via the `onMouseMove` event handler and stores them in state with `useState`, while `useCallback` memoizes the handler for efficiency. The circle’s center is rendered at the cursor’s `clientX` and `clientY` coordinates, with a fixed radius of 30 pixels, using React and ReactDOM to manage the SVG output.