BeginEdit cloned Document eagerly but Record lazily from record.Document,
so a cache change in place before Record's first read made the two copies
come from different states. BeginEdit now serializes the document once and
deserializes Document from those bytes; Record deserializes the same bytes
on first read. Record is the state the edit began from, and costs one
deserialize only when a drawer reads it, with no second serialize.
Each commit still spends the edit, so a slider drag begins an edit per
frame; that serialize was already paid for Document, and Record adds a
deserialize only in frames where a drawer reads it.
The new test mutates the cached document between BeginEdit and the first
Record read; it fails on the lazy clone.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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