revealed, that the "is_expanded" parameter is not passed to the
"_update_icon_for_nid" method from "_update_icon".
Instead the expanded state is read by calling "_get_node_data".
But when collapsing a node, the node data does not seem to be updated.
Only when expanding a node, the icon changes but it never changes back
when collapsing the node.
Since I don't know whether the "is_expanded" parameter should be passed
or the node data should be updated instead, or both, I cannot submit a
patch. Could someone please have a look at this?