@shivanigithub this could also help with top-level origin. That is, through a document's browsing context's container document we can get to the actual parent of a document. If we keep doing that we hit the top-level document. We can then get the true top-level origin that's not tainted by bfcache.

In particular, if you have document A1, which nests A2, which nests A3. And then you navigate A2 to B. And then you navigate A1 to C.

If you go through A1's (or B's) parent documents using the above pattern you'll get A1. If you use top-level browsing context's active document you'll get C, which is bad.

Oh, shoot. I think I was under the mistaken impression that the nested browsing context stays set, even after an iframe gets removed. So a lot of my comments are wrong, from that perspective, and this PR does not make normative changes in the way I thought it did.

I think the main thing would be to straighten out the child BC definition, and then I can take another look at this with the correct frame of mind.

I'm not really sure what you're asking for at this point.

I've resolved all the comments that were resulting from my confusion and added an idea on how to make it less confusing.

Proposed commit message:

Give browsing context an explicit container

This generally sets out the infrastructure for a more secure model for working with browsing contexts, as outlined in #5020.

* Adds browsing context's container
* Adds browsing context's container document
* Makes "update the rendering" account for browsing contexts in shadow trees
* Removes nested through; closes #4409.

I think #1336 is also fixed now?

(Note to self: rebase #5095 once this lands.)