To give slightly more technical detail, it's nsObjectLoadingContent::HasNewFrame that posts the nsAsyncInstantiateEvent to the event loop when the embedding element is reflowed (at least in the case when the embedding element is an <object>). HasNewFrame is currently only called from nsObjectFrame::DidReflow.
To give slightly more technical detail, it's nsObjectLoading Content: :HasNewFrame that posts the nsAsyncInstanti ateEvent to the event loop when the embedding element is reflowed (at least in the case when the embedding element is an <object>). HasNewFrame is currently only called from nsObjectFrame: :DidReflow.