A React and D3 example demonstrates how to track and render the mouse position as an SVG circle that follows the cursor across the full viewport. The code uses React’s `useState` and `useCallback` hooks within an `App` component, handling `onMouseMove` events on an `<svg>` element. The circle’s coordinates update directly from `clientX` and `clientY`, with all dimensions set to `window.innerWidth` and `window.innerHeight`. No external data is loaded; the visualization relies solely on user interaction and SVG rendering.