A circle inside an SVG follows the mouse's movement across the viewport in real time. The app initializes the circle at the center of a 960-by-500 coordinate space, then updates its position on every mousemove event. React’s `useState` and `useCallback` hooks manage the mouse coordinates, which are passed to the circle’s `cx` and `cy` attributes. The example renders using ReactDOM and React 16.13.1, with the SVG’s `onMouseMove` handler triggering the state update.