The reason was a synchronization issue.
Actually "LayerPropertiesNodeRef" is not a reference, but
a mirror copy of the LayerPropertiesNode the reference points
to. Changes to the original must be synchronized into the
reference and back.
This concept is a tribute to the original implementation and
the node reference was a convenience add-on to the iterator-
based API.
Better solution in general is to replace the LayerPropertiesNodeRef
concept with a real reference.
0285a5195c