Yeah, that is an internal property for when the `targetPath` has been set before the scene is fully ready.  The app developer can set a default path, which is set, but the node doesn't yet exist, so it can't set up the event handlers for when the target node's `transform` changes.  It instead sets `needToSetupEventHandler$` to `true` which is checked in `onSceneReady$` (and the event handlers are set up then, if appropriate).