Problem with parent page slug only in WordPress admin

Edit. Here’s another solution to not mess with the edit slug button.

The function get_sample_permalink_html in wp_admin/includes/post.php outputs the sample permalink and an edit button. It can be filtered like this:

add_filter('get_sample_permalink_html','my_sample_permalink',10,2);
function my_sample_permalink ($page_link,$id){
    $page = get_page($id);
    if($page->post_type == "page" && $page->post_parent) {
        $parent = get_page($page->post_parent);
        $page_link = preg_replace("/(sample-permalink\">).*?(<)/","$1".home_url("https://wordpress.stackexchange.com/").$parent->post_name."/$2",$page_link);
    }
    return $page_link;
}

Can’t say I like this solution but maybe it works for you.