At least Gecko and Blink use a list of trees to represent session history, not a joint list of session histories in browsing contexts (since as far as I see, the spec way to define this all can't handle iframes in the unloaded, yet in session history, pages properly).

http://searchfox.org/mozilla-central/source/docshell/shistory/nsISHTransaction.idl
https://cs.chromium.org/chromium/src/content/renderer/history_controller.h?l=59

Dynamic changes to the dom where iframes are added or removed are still a tricky case.
Gecko and IE/Edge might handle them a bit better than Blink/webkit as http://mozilla.pettay.fi/moztests/history2/Start.html hints (blink ends up loading an url from session history to an iframe which never had that url loaded before).