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I started with a class diagram with 20+ classes all in the default package. I created a package hierarchy and moved the classes (dnd via the project window) into the package tree. Then attempted to relayout using the Orthogonal layout. It caused a ConcurrentModificationException. Note that while I was moving the classes to the package(s), the class diagram showed many, many layout artifacts (not all relationships rendered, spurious lines and dots, etc.)
Created attachment 31476 [details] runtime log of ConcurrentModificationException on layout
no longer reproducible witht the final version of NB 5.5.