A full-screen SVG circle follows the mouse cursor in real time. The visualization demonstrates basic pointer tracking within a D3.js v7 application, where the circle updates its `cx` and `cy` attributes directly on every `mousemove` event. The code selects an SVG container from a `#app` div, appends a circle with a fixed radius of 50 pixels, and binds the event listener using D3's `selection.on` method. The body has no margins and hides overflow, ensuring the circle moves across the entire viewport without scrollbars.