I am wondering how Back, Forward browser button works for iframes. Does it bring back/forward iframe or top window?

I remember I had before a problem because I wanted to bring back iframe and not top window as it did. But today I tested my Facebook app and was surprised to see it works as I need which means it brings back/forward iframe window and not top. Is it possible that Facebook has some javascript code which does that or is this normall behaviour?

Thank you