The following suggestions should help you when you need to troubleshoot the SchemaWidget.
Use the logging system
The widget logs various information. Make sure you turned the logging level to debug or trace to get more information.
Display debug markers in the schema
In SchemaWidgetViewImpl, there is a method called "drawDebugMarkers". Search for the line in the constructor with a comment containing the call to this method, and uncomment. The widget will then display various markers (the size of the central contents and their center point, the view node's center point, the base viewport, ...). Those markers might be helpful for debugging issues for example related to the manipulation of the widget's view.
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AnnekeWalter - 20 Feb 2014