oncontextmenu event browser event is disabled in dhtmlxmenu.js. You may try to locate and comment the following lines to enable it:
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this.base.oncontextmenu = function(e) { e = e || event; e.returnValue = false; return false; }
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m.oncontextmenu = function(e) { e = e || event; e.returnValue = false; return false; }
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s.oncontextmenu = function(e) { e = e||event; e.returnValue = false; e.cancelBubble = true; return false; }
commenting: browser standard popup BUT the shortcut to open the link in a new tab is not showing…it seems the browser is not able to recognize the link