dropdown of latest chrome(ver 53) does not accept dispatchEv

Question

Anyone has experience the issues on dropdown control in latest Google Chrome(ver 53) where you can't simulate mousedown event on Dropdown control through dispatchEvent(). 

var event = document.createEvent("MouseEvent");

event.initMouseEvent("mousedown", true, true, window);

document.getElementById(targetcontrol).dispatchEvent(event); 

How can i suppose to do for fix it?