Hai ragione.

Main
codice:
TextArea_D = document.getElementsByClassName('Att_TextArea_DIV');
Pos = 0;

if(Is_Object_In_Object(window.getSelection().anchorNode,TextArea_D[Pos])){
  /* Div attivo */
}
Alcune funzioni
codice:
function Is_Object_In_Object(Object1,Object2){
	
	IN = false;
	if(Defined(Object1)){
		while((Object1 != Object2) && (Defined(Object1.parentNode))){
			Object1 = Object1.parentNode;
			if(Object1 == Object2)IN = true
		}
	}
	return IN;

}

function Defined(Var){
	return ((typeof(Var) != 'undefined') && (Var != null)) ? true : false;
}