godot/editor/icons/SubViewportContainer.svg

<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><g fill="none" stroke="#8eef97"><rect width="12" height="12" x="2" y="2" stroke-width="2" rx="1"/><rect width="6.75" height="6.75" x="4.625" y="4.625" stroke-width="1.25" rx="1.5"/></g></svg>