… initial threshold. (servo#45680)
These changes both make our threshold indexing code easier to follow and
also align us better with how other browser engines have implemented
IntersectionObserver. There are spec issues that have been open for six
years without resolution, sadly, but the relevant WPT tests help us
ensure that we're matching other engines.
Testing: Newly passing tests.
Fixes: part of servo#35767
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Signed-off-by: Josh Matthews <josh@joshmatthews.net>