I have just noticed a little problem I have been having. I wasn’t sure where it was coming from, but I know now.
While you look at the preview of a Nifty XML file, if you open the SceneViewer (bring it to the front), it will somehow fubar the preview of the GUI. Everything is rendered halfway through the right side of the preview screen. Closing and reopening the XML file seems to fix the problem. It is repeatable here.