code: func (a *Application) SetRoot(root Primitive, fullscreen bool) *Application {
  	a.Lock()
  	a.root = root
  	a.rootFullscreen = fullscreen
  	if a.screen != nil {
  		a.screen.Clear()
  	}
  	a.Unlock()
  
  	a.SetFocus(root)
  
  	return a
  }
docs: // SetRoot sets the root primitive for this application. If "fullscreen" is set
  // to true, the root primitive's position will be changed to fill the screen.
  //
  // This function must be called at least once or nothing will be displayed when
  // the application starts.
  //
  // It also calls SetFocus() on the primitive.