on_context

a property used to define custom context-click (right click) handlers for elements in widgets

Example

var dtable = webix.ui({
    view:"datatable",
    data:[
        { text:"<div class='myActive'>click here</div>" }
    ]
});
dtable.on_context.myActive = function(id, event, target){
    webix.message("Active area was right-clicked");
};
//or
dtable.on_context["myActive"] = function(id, event, target){
    webix.message("Active area was right-clicked");
};

It's a object property that contains already available handlers.
All the handlers (existing and newly-added) are specified by the related class name.