This example displays a single circle that follows the mouse cursor across the full browser window. The circle is rendered as an SVG element with its position updated on every mouse movement event. Built with React, the code uses the `useState` and `useCallback` hooks to manage and optimize the cursor position state, while `ReactDOM.render` mounts the component. The SVG viewport is set to the window’s dimensions, and the circle’s center coordinates are directly bound to the tracked mouse position.