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.
