Dependency on undefined behavior via getBoundingClientRect()

[frivoal]

As discussed in w3c/csswg-drafts#4463, it isn't clear what is supposed to happen with getBoundingClientRect() when various fragments are spread over multiple pages. See that issue for details. Since UAs that support scripting and pagination at the same time exist, this needs an answer.

This is not an issue created by IntersectionObserver, but since IntersectionObserver's behavior is defined in terms of getBoundingClientRect(), it is an issue that it inherits.

I expect this to be something that first has to be sorted out in the definition of getBoundingClientRect(). However, I think it is worth raising here as well because:

• Even though it is an problem in a normative dependency, until it is solved, IntersectionObserver has the same issue.
• People interested in IntersectionObserver may be interested in getting involved in that issue and advocating for one solution or another.
• Once that issue is solved, depending on how it is solved, IntersectionObserver may or may not need adjustments to handle that case.

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Note: this is not a duplicate of #10 and #13: those were concerned about dealing with fragmentation in general, and that has been answered by using getBoundingClientRect(). This is concerned with the cases where getBoundingClientRect() itself is (currently) undefined.