The physical depth is the maximum elevation that the Window allows.
Physical layers drawn at or above this elevation will have their elevation clamped to this value. This can happen if the physical layer itself has an elevation larger than the available depth, or if some ancestor of the layer causes it to have a cumulative elevation that is larger than the available depth.
The default value is double.maxFinite, which is used for platforms that do not specify a maximum elevation. This property is currently only expected to be set to a non-default value on Fuchsia.
final double physicalDepth