Off by onebeginner
For any range, three values per edge — one below, the edge itself, one above — catch every single-character mistake it is possible to make in the comparison.
Off-by-one is the most common bug there is. Test on the line and both sides of it.
Your suite passes a correct isEligible and fails all four off-by-one variants.
Published in full, before you start — every point is one of these and there is nothing else. Each one runs your code; it is not a search for keywords.
For any range, three values per edge — one below, the edge itself, one above — catch every single-character mistake it is possible to make in the comparison.