The first target's EntityId. Populated for every target kind — Unity objects and ECS components.
Prefer this over targetObject.GetEntityId() when the target may be an ECS component with no backing Object. Returns EntityId.None only when the SerializedObject has no targets.
For ECS-component targets, UpdateIfRequiredOrScript does not auto-detect external chunk mutation until the chunk change-version accessor is wired. Call Update explicitly to pick up externally-driven changes.
Additional resources: targetEntityIds, targetObject