I've applied to be an Observer but as yet I can't post to the issue.
Thx for the project code, I see now what's happening.
If you exit the page with an <h:command link, it calls the bean's PreDestroy method. If you exit any other way, like <a href= or reload the page or type something into the browser's location bar and load that, then it doesn't call the PreDestroy method. I haven't tried killing the session yet.
I think in my case I would run the risk of accumulating dead diagrams which can't be displayed and can't be garbage collected because the data model still has refs to their listeners, and that would be a problem.
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