Yeah, that is great, thank you! So, it seems like the `<aside>` is `position: absolute`, and it has `top: auto`, so we should be using its hypothetical position. Instead, it seems we're using `top: 0` and positioning it relative to the first page. I'm not sure how we handle hypothetical positions when fragmenting, but yeah, this looks clearly like a bug. Her's a more reduced test-case based on your example.
Bug 1569436 Comment 9 Edit History
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(duplicate comment, sorry)