it is caused by
// Synchronizing the tinymce width with the content width
add_filter(‘tiny_mce_before_init’, ‘nirvana_dynamic_editor_styles’, 10);
function nirvana_dynamic_editor_styles($settings){
$settings[‘content_css’] .= “,”.admin_url(‘admin-ajax.php’) .”/?action=dynamic_styles”;
return $settings;
}
from nirvana/admin/admin-functions.php (in WP4.8 there were changes in TinyMCE AFAIK – perhaps Nirvana need a fix)