Closes #15027 and #14995

When the target element is inside a block or component, get_following_sibling_elements was looking only for sibling elements after the block/component. This PR fixes it by starting the search from the begging of the block/component and skipping nodes that go before the target element.

Also, I noticed that selector like .elem:has(+ .not-adjacent-sibling) isn't marked as unused. Example. Though, it's false negative and isn't that bad.

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