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…tion root The spec clips overflow ancestors *until* the root, then intersects with the root intersection rectangle, which includes rootMargin. We were clipping the target to the root's own overflow (ScrollView, overflow: hidden) first, so rootMargin expanded rootBounds but not the intersection area. Skip overflow clipping on the specified ancestor for IntersectionObserver. Intermediate clippers are unchanged.
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Summary
Hit this using IntersectionObserver on a ScrollView with
rootMarginto preload content below the fold.rootBoundsgrew by the margin, butintersectionRatiostayed 0 for anything past the clip.We're clipping the target to the root's overflow (the ScrollView /
overflow: hiddenbox) before intersecting with the margin-expanded rect. That's not what the spec does.Compute the intersection: walk containing blocks while container is not root and clip there, then intersect with the root intersection rectangle, which already includes
rootMargin. The spec also calls out thatrootMarginonly applies to the root — clipping by any other ancestor is unchanged.This stops overflow-clipping the observation root itself. Intermediate clippers stay as they are, so a viewport-rooted observer still can't see past a ScrollView. That's
scrollMargin, which we don't implement.--
Lots of words to say: This enables rootMargin to correctly extend the viewport of ScrollViews or any other view with overflow clipping when using that view as the root. This also gives users a path around missing scrollMargin.
Changelog:
[GENERAL] [FIXED] - IntersectionObserver rootMargin now affects intersection against a clipping root
Test Plan
rootMargin: 150px+threshold: [0.01]— target fully below the fold reports intersecting