A single circle follows the mouse pointer across a full-screen SVG. The example demonstrates basic D3 event handling and shape manipulation, using a selection created with `d3.select` and attribute updates on the `mousemove` event. The circle’s `cx` and `cy` coordinates are directly set to the event’s `x` and `y` values, with no animation or interpolation. The SVG canvas is sized to the browser window via `window.innerWidth` and `window.innerHeight`, and the CSS hides page margins and scrollbars.