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At the beginning of a new chunk we may be trying to complete a UTF-8 sequence started in the previous chunk (contained in the `undecode0` buffer). If it turns out to be invalid, we must apply the `onErr` handler to every character in that buffer. When we reach the end of the chunk, we must also be more careful about when to keep the previous buffer: a UTF-8 sequence (up to 4 bytes) can span more than two chunks, when those chunks are very short (of length 0, 1, or 2).
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