function canvasWindow(existingCanvas: CanvasScript){
    var root = existingCanvas.getElement('root');
    var ui = @:privateAccess existingCanvas.cui;
    root.y += 10; // Move stuff down a bit to make place for header
    addHeader(ui);
}

function addHeader(ui: Zui){
  ui.button(... // add head element, close button, etc.. as usual way
}