onBeforeColumnDrag

fires the moment you start dragging the column from its source position

void onBeforeColumnDrag(string sourceId,Event object event);

sourceId	string	the source column id
event	Event object	the native event object

$$("datatable1").attachEvent("onBeforeColumnDrag", function(sourceId, event){
    // some code
    return true;
});

Details

The event is blockable. Returning false will prevent column dragging.