Well, I have added "flicker prevention" for the main menu, I guess it might be the cause. Can you try with

void CocoMenuBar::Set(Event<Bar&> bar)
{
	if(lock) return;
	lock++;
	[cocomenu removeAllItems];
	bar(*this);
	lock--;
}

(I do not see any obvious reason now why it should not work, but at least this might narrow it down).