The dhtmlxMenu doesn’t provide public event for such purpose, but it has inner method which called onMouseOver and can be used for your purpose
in dhtmlxProtobar.js you can locate next code
dhtmlXButtonPrototypeObject.prototype._onmouseoverX=function(e){
if (!e) e=event;
custom_function(this.objectNode.text);
if (this.timeoutop) clearTimeout(this.timeoutop);
this.timeoutop=setTimeout( this.objectNode._delayedTimerCall(this.objectNode,"_onmouseoverY"),50);
};
text marked bold - possible addition, the custom_function will be called on each mouseover event
Itwork~! Thank you!