>> Please note that in the code posted 3 days ago I used the hook that you
>> proposed recently window-state-change-functions, so this code is not yet
>> functional :)
>
> Ahh.. That hook has not been written yet and I'm not sure whether I'll
> be able to ever do that.  In either case: The window state will not
> change when a window gets just reused by 'display-buffer' and no
> selection takes place.  So can't you simply use 'post-command-hook'
> for your purpose?

The implementation of window-state-change-functions posted to bug#32672
uses post-command-hook.  It makes possible display-buffer-directionally,
e.g. S-M-up ffap RET will open a file link in the upper window, etc.