I've noticed that the onpropertychange event sometimes is flaky between browsers.
Because oninput and onchange is unavailable in the Custom Events control, try adding the event in the Body Custom Events of the form page itself on the Behavior tab.
add an onload event and adjust the attributes of your dropdown box like this:
var ddNewLoc = document.getElementById('DropDownNewLocation');
ddNewLoc.setAttribute('oninput','updateDrop();showSave();');
oninput has worked pretty well for me in this regard. you could also try onchange if that doesn't do it for you.
in my example script above, any referenced functions added to "oninput" should go in the "Script" field of the Behavior tab for the form builder. you should also be able to just write out the script between those apostrophes. let me know if you need a hand.
here's some additional reading on (and examples of) this method:
http://atmaworkflow.com/2014/10/24/skirting-the-form-builder-components-javascript-onchange-event-reservations/
http://atmaworkflow.com/2014/11/17/workflow-short-oninput-function/
http://atmaworkflow.com/2014/12/15/highlight-element-by-checkbox-state/