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Caching in this way assumes that the pure_height of an Item is
independent of where the line-breaks are.
Anything that gets pure_height from its neighbors would break that
assumption, but now it seems there are no such Items, as they all use
pure-from-neighbors-interface to set their extra-spacing-height.
Tied accidentals break that assumption as well, but that's not our
fault. I'll write a cautionary comment in a separate commit.