I would like to ask for input on the use of or if it is a top-level browsing context whose session history contains only one Document in https://html.spec.whatwg.org/multipage/#script-closable
in the context of current work in progress at servo/servo#20865 (comment)

It does seem that the 'easy' thing to do would be to use #dom-history-length, however that is the length of the joint-session-history, while #script-closable writes about the 'session-history'.

Would it be acceptable to use the length of the history of the window that is about the be closed, thereby actually using the joint-session-history length, as opposed to the session-history length that is mentioned by the spec? I guess using the joint history actually narrows down the concept a bit...

If it is really the session-history that should be used, how would one go about checking it's length?
The spec does tell us that A browsing context's session history consists of a flat list of session history entries.(https://html.spec.whatwg.org/multipage/#the-session-history-of-browsing-contexts), and perhaps it could use an additional normative section on how to determine it's length, or even a specific concept of 'contains only one document' to which #script-closable could simply link?

Or should #script-closable be updated to simply use the length of the history object of the current window?