former enumerates children in traversal order, and the latter enumerates
the same children in the hit test order. The two lists must have the same
length and contain the same ids. They may only differ in the order the
ids are listed in. For more information about different child orders, see
<code>DebugSemanticsDumpOrder</code>.</p>
<p>The system retains the nodes that are currently reachable from the root.
A given update need not contain information for nodes that do not change
in the update. If a node is n