A full-window SVG renders a single rounded rectangle that fills the entire viewport. The example demonstrates a minimal D3 setup, using `select` to target the `svg` element and setting its `width` and `height` attributes from `document.body.clientWidth` and `clientHeight`. A `rect` is appended with matching dimensions and a corner radius of 40 pixels. The SVG is styled with fixed positioning and hidden overflow, creating a blank canvas with no data or interaction.