A circular SVG marker tracks the cursor's position in real time across a full-browser canvas. The visualization uses React's `useState` and `useCallback` hooks to manage mouse coordinates, with an `onMouseMove` event handler updating a circle element centered at the pointer. Built with React 16.9 and rendered as SVG, the example demonstrates a minimal interactive pattern where the circle's `cx` and `cy` attributes are bound to the current mouse position, creating a responsive follower effect without additional data sources or complex D3 logic.