function bind_props(props_parent, props_now) {
	for (const key in props_now) {
		const initial_value = props_parent[key];
		const value = props_now[key];
		if (
			initial_value === undefined &&
			value !== undefined &&
			Object.getOwnPropertyDescriptor(props_parent, key)?.set
		) {
			props_parent[key] = value;
		}
	}
}
Legacy mode: If the prop has a fallback and is bound in the
parent component, propagate the fallback value upwards.
@param {Record<string, unknown>} props_parent
@param {Record<string, unknown>} props_now