Add a test that creates two observers watching the same target inside
an overflow:hidden container: one without scroll margin and one with
scrollMargin "50px". The target is partially visible (10px out of 50px).
Without scroll margin, the ratio is ~0.19 (10px visible / 50px target).
With scroll margin, the expanded scrollport should make the full target
visible (ratio=1), but currently both report the same ratio because
scroll margin is not yet applied in compute_intersection.
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