A single dark circle follows the mouse cursor across a blank SVG canvas. The circle’s center updates to the pointer’s client coordinates on every mouse move, creating a direct and immediate tracking effect. The example uses React’s `useState` and `useCallback` hooks to manage and update the circle’s position, while rendering occurs through an SVG `<circle>` element. D3’s `arc` function is imported but not used, and the code relies on ReactDOM to mount the interactive component.