A record representing the layout state of a
child view. It is runnable as a task on another
thread. All access to the child view that is
based upon a read-lock on the model should synchronize
on this object (i.e. The layout thread and the GUI
thread can both have a read lock on the model at the
same time and are not protected from each other).
Access to a child view hierarchy is serialized via
synchronization on the ChildState instance.