A red arrow with a white fill follows the mouse cursor across a full-window SVG canvas. The arrowhead is defined in a marker with a custom path, and the arrow's curved path is calculated from the cursor's x- and y-coordinates. The example demonstrates unidirectional data flow using the d3.select and .join methods, where mousemove events trigger setState to update the state, and a new path attribute is rendered each time. It uses d3.v7 from unpkg and renders exclusively as SVG.