The solution proposed in inital anser ( at least it worked locally, but based on your dhtmlxMenu version you probably need to create one more modification.
In dhtmlxMenu.js
dhtmlXContextMenuObject.prototype._contextStart=function(obj,e){
…
if ((!e)||(e.button!=2))
This line need to be replaced with
if ((!e)||(e.button!=1))
If problem still occurs for you - please contact directly at dhtmlx@scand.com and provide your customer number - I will send you working sample.