Version GXT 3.0.0.b
Class com.sencha.gxt.widget.core.client.Component
method
Code:
  
protected void onResize(int width, int height) {
    if (mask) {
      mask(maskMessage);
    }
  }
but method
Code:
  public void mask(String message) {
    if (!mask) {
      mask = true;
      maskMessage = message;
      getElement().mask(message);
    }
  }
So if I resize componet I don't see mask.