A single gray circle follows the mouse cursor across an SVG canvas, creating a direct and responsive visual interaction. The example demonstrates how React manages the circle's position through state, with an `onMouseMove` event handler updating the cursor coordinates on every movement. D3's `d3.min.js` library is loaded but not used for any calculations or rendering, as the visualization relies entirely on React's `useState` hook and ReactDOM to draw the SVG circle. The full-screen, borderless layout keeps the focus on the smooth, real-time tracking of the pointer.