stacking multiple panels in z-order?

Now that the Canvas widget is available, I'd like to be able to create a panel which holds a number of subWidgets in a specified z-order, and in which multiple widgets are visible simultaneously. For instance, an HTML widget with a transparent background, a Canvas with translucent highlights drawn

FormPanel.SubmitCompleteHandler called even on error return?

I find that even when my server sends back 400 and 401 error responses in response to a FormPanel submit, the SubmitCompleteHandler of the FormPanel is still being called. Is there any way in a SubmitCompleteHandler to see if the submit actually succeeded or not? This is with GWT 2.1.1. -- You r