> What an instance side preference would mean ?
> A different preference for every instance ?

There is only one instance of SystemNavigation.

> My opinion is that they belong to class only.

I suppose I could add the first class-var to SystemNavigation.
However, SystemNavigation is a Singleton _instance_ which is where its
other state and API are.  So we would have this lone #traceMessages
option on the SystemNavigation class-side just so we could use a
pragma.  It creates a dichotomy about which SystemNavigation is
supposed to be used.  Since you were only suggesting anyway, the
easiest thing for now would probably be to just leave it in
Preferences.

 - Chris