anchor.diff:  
     @override
     S find<S>(Offset regionOffset) {
  -    final Iterable<S> all = findAll<S>(regionOffset);
  -    if (all.isEmpty) {
  -      return null;
  -    }
  -    return all.first;
  +    if (link.leader == null) {
  +      return showWhenUnlinked ? super.find<S>(regionOffset - unlinkedOffset) : null;
  +    }
  +    final Offset transformedOffset = _transformOffset<S>(regionOffset);
  +    return transformedOffset == null ? null : super.find<S>(transformedOffset);
     }
   
     @override
  
positive.code: 
    @override
    S find<S>(Offset regionOffset) {
      final Iterable<S> all = findAll<S>(regionOffset);
      if (all.isEmpty) {
        return null;
      }
      return all.first;
    }
  
    @override
  
negative.code: 
    @override
    S find<S>(Offset regionOffset) {
      Layer current = lastChild;
      while (current != null) {
        final Object value = current.find<S>(regionOffset);
        if (value != null) {
          return value;
        }
        current = current.previousSibling;
      }
      return null;
    }
  
    @override
  