A React application renders an SVG circle that follows the mouse cursor across a 960x500 canvas. The component uses the `useState` hook to track mouse position and `useCallback` to memoize the mouse-move event handler, preventing unnecessary re-renders. The `onMouseMove` event on the SVG element updates the circle's `cx` and `cy` attributes via D3-style coordinate mapping. The demo showcases React 16.9 hooks for state management and D3 v5 for SVG rendering, with the circle's position initialized at the canvas center before responding to user interaction.