A yellow circle follows the mouse cursor across an SVG canvas, with its position updating in real time as the pointer moves. The visualization combines React's state management with D3's geometric utilities, using `useState` and `useCallback` hooks to track mouse coordinates via `onMouseMove` events. D3's `arc` function is imported, though only a basic SVG circle element renders the follower. The app is built with React 16.9 and D3 v5, mounting directly to the DOM root.