Hi, I had post this question at 'http://www.scbr.com/docs/pr

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

    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.