I have a simple Window which is rendering incorrectly. (Clarifying details below.)
A quick search of SmartGWT's forum yielded:http://forums.smartclient.com/showthrea ... tle+bottom
That is not my post, but it describes the same problem I was having. The recommended fix worked, so it seems you should be generating addItem calls when adding to a Window.
I should clarify - when actually running the change worked. However, GWT Designer is not rendering addItem calls.