We can lazily compute some rectangles in IntersectionObserver when we know that we're going to fire an observation.
<rdar://problem/113083595>
Pull request: https://github.com/WebKit/WebKit/pull/16214
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Committed 266458@main (39a323e0f736): <https://commits.webkit.org/266458@main> Reviewed commits have been landed. Closing PR #16214 and removing active labels.