[fix] properly scroll if body has margin

[bluwy]

Fixes #2760

Changes

• Fixed my dumb tests update mistake from my last PR (tests were actually failing)
• Use document.documentElement instead of document.body when checking for max scroll y
• Also fixes another sizable bug I found: When scrolling to/near the end of a page, the new page after navigation would also be scrolled to/near the end of the page by the browser regardless of the new page scroll height. Added extra checks for that.

Note: We may need to find a robust way to check for scrolling before the page onMount() runs. The pageYOffset trick is starting to bite us.

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Tests

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[bluwy]

@benmccann I made quite a big change since your review. The main takeaways are noted in the PR description, and in the code comments. I feel like the implementation is starting to get hacky, but unless we find a better way, we may have to stick with it for now.

[gavinr]

Thank you @bluwy for this PR. I really appreciate your work on this project. Can I help this PR to land by testing or anything? Or are we just waiting on a review?

[bluwy]

Thanks @gavinr. Yeah I'm currently waiting for Ben's review as he has more experience with the router. If you can help test this branch, that would be nice as well to bring more confidence in the PR.

[gavinr]

I have tested this with the reproduction case in #2760 and it does resolve that issue. Thank you!

[Rich-Harris]

niiiice, amazing work

[antony]

game changer! @Wolfr check this.