sets the position of the window relative to the the screen
webix.ui({
    view:"window",
    position:"center",
    // window config
}).show();
The positioning function takes the default window position as a parameter and allows modifying its aspects.
webix.ui({
    view:"window",
    head:"",
    body:{...},
    position:function(state){ 
        state.left = 20; // fixed values
        state.top = 20;
        state.width -=60; // relative values
        state.height +=60;
    }
});
The default window position is a top left corner of the viewport.