Implement the containing block chain traversal in compute_intersection
(steps 2-3 of the spec algorithm). Walk from the target's paintable box
up through the containing block chain, clipping the intersection rect
against each ancestor that has a content clip (overflow != visible).
This fixes the case where a target is inside an overflow:hidden or
overflow:clip container and pushed below the visible area. Previously,
the intersection ratio was incorrectly non-zero because we only
intersected with the root bounds (viewport), not intermediate
scroll containers.
Also update the scroll container clipping tests to verify the
intersection ratio (which is what compute_intersection affects)
rather than isIntersecting (which per spec is based on targetRect
vs rootBounds, not the clipped intersection rect).
Add tests for the implicit root case where targets are inside scroll
containers with various overflow types:
- overflow:hidden container with target pushed below visible area:
should NOT be intersecting (currently wrong, shows intersecting).
- overflow:clip container with target pushed below visible area:
should NOT be intersecting (currently wrong, shows intersecting).
- overflow:hidden container with target at top (visible):
should be intersecting (correct).
The two failing tests currently reflect wrong behavior because
compute_intersection does not clip against intermediate scroll
containers when walking the containing block chain.
