A diagnostic's span often reaches past the block a split moved — most commonly
to a hook dependency array, which legitimately grows when closure variables
become props. Requiring every line of the span to match contiguously reported
the moved body as new.
The block must still start at the same line in the base and appear in order,
and >=90% of it must be present. Genuinely new code shares neither the anchor
nor the ordering.
Neil
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