What is props.children really?

interface AutoScrollProps {
    children: JSXElement
}

function AutoScroll(props: AutoScrollProps) {
    onMount(() => {
        if (props.children instanceof HTMLElement) {
            if (document.body.contains(props.children)) {
                console.log("Element is mounted")
            } else {
                console.log("Element is not mounted")
            }
        }
    })

    return props.children
}

/* ... */

<AutoScroll>
    <div />
</AutoScroll>