system-uicons/src/images/icons/fullscreen.svg

<svg height="21" viewBox="0 0 21 21" width="21" xmlns="http://www.w3.org/2000/svg"><g fill="none" fill-rule="evenodd" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" transform="translate(2 2)"><path d="m16.5 5.5v-4.978l-5.5.014"/><path d="m16.5.522-6 5.907"/><path d="m11 16.521 5.5.002-.013-5.5"/><path d="m16.5 16.429-6-5.907"/><path d="m.5 5.5v-5h5.5"/><path d="m6.5 6.429-6-5.907"/><path d="m6 16.516-5.5.007v-5.023"/><path d="m6.5 10.5-6 6"/></g></svg>