> > Basically "nibtool --classes MainMenu.nib" returns the same as in the
> > classes.nib, but you can also return all the objects, the hierarchy of
> > the  objects and all connections between them.

Thank you for your help. nibtool says it can compare two nibs, but I
did not find how to do that.
I ran nibtool with -all option on both nib files, and there were nearly
hundred of objects, mainly because each menu has its associated object
apparently. And the objects did not have the same object number in both
files, so it was hard to compare unless you know what you are looking
for.

I did find that the window causing trouble was associated with the
default NSView in my file, and with a custom class in the working file.
So this is definitely a helpful tool. Thank you