imgIRequest
status_load_partial 0x2 used internally by imgrequest to flag that a request is being cancelled as a result of a failure of a proxy holder and not an internal failure.
...regardless, there's no reason for this flag to be public, and it should either go away or become a private state flag within imgrequest.

imgIDecoderObserver
by the time this callback is been called, the size has been set on the container and status_size_available has been set on the associated imgrequest.