+/** @return Pair: TimeAxisView, layer index.
+ * TimeAxisView is non-0 if this object covers y, or one of its children does.
+ * If the covering object is a child axis, then the child is returned.
+ * TimeAxisView is 0 otherwise.
+ * Layer index is the layer number if the TimeAxisView is valid and is in stacked
+ * region display mode, otherwise 0.
+ */
+std::pair<TimeAxisView*, layer_t>