The
"Default" deselection guard. It disallows deselection of a
default actor node, because such a node cannot really
be deselected in any case. If the track is
unpinned, then it also disallows the deselection of all but base candidate and
non-participant nodes. Deselection will cause the unpinned track to drop to its
default state in which only the base peer board is displayed. The user may easily
reselect a base candidate or non-participant in that state, while other nodes
might require a climb back up the tree.