eventPos

the top (vertical resizing) or left (horizontal resizing) absolute event position

Example

var parentContainer = document.getElementById("resizeDiv");
 
webix.event(parentContainer,"mousedown",function(e){
 
  var resizeStick = webix.ui.resizearea({
       container:parentContainer,
       dir:"y",
       eventPos:webix.html.pos(e).y,
       height: 5,
       start:webix.html.pos(e).y-webix.html.offset(parentContainer).y,
       width: parentContainer.offsetWidth,
   });
 
})

Setting a pure number value doesn't make any sense.