The elevation adjustment that the parent imposes on this node.
The elevation property is relative to the elevation of the parent
SemanticsNode. However, as SemanticsConfigurations from various
RenderObjects are merged into each other to form that
SemanticsNode the parent’s elevation may change. This requires an
adjustment of the child’s relative elevation which is represented by this
The value is rarely accessed directly. Instead, for most use cases the elevation value should be used, which includes this adjustment.