Yes indeed, but up to now I’ve been unable to find this error, something that has an undesirable effect only in macOS 15, none in the two earlier versions of macOS that I could try. Of course I compared very carefully, and more than once, the code of the bare-bones Goperf with the corresponding parts of Goperfekt and couldn’t find anything wrong. It might be something simple, but not too obvious. I’ll continue the search. Thanks for your feedback.

I tried adding to my document class in my bare bones version Goperf, a definition of observeValueForKeyPath:ofObject:change:context: (from the NSKeyValueObserving protocol), with no content at all. This sole addition produces the bug !!! So THIS METHOD DEFINITION is the problem. The presence or absence of the addObserver:… statement in the init method makes no difference. Again, WOW!

Hopefully the happy end of this saga: a very helpful German programmer, Mr. Christian Tietze (https://christiantietze.de), who had first given me the idea of the bare bones reduction of my app, suggested yesterday that I try transferring the observing task I mentioned from my document class to another class. This turned out to be real easy to do and, lo and behold, now everything seems to run just as it was meant to. Heartfelt thanks to the whole Mac programming community!