A circle follows the mouse cursor inside an SVG canvas, updating its position in real time as the pointer moves. The example combines React state management with D3.js v7 for SVG rendering, using the `useState` hook to track mouse coordinates and a `useCallback` event handler to update them on `mousemove`. The circle is drawn with hardcoded width and height values, and the mouse position is captured via `clientX` and `clientY` from the native browser event.