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`xkb_keysym_to_utf8`: Require only 5 bytes for the buffer, as UTF-8 encodes code points on up to 4 bytes + 1 byte for the NULL-terminating byte. Previous standard [RFC 2279](https://datatracker.ietf.org/doc/html/rfc2279) (1998) required up to 6 bytes per code point, but has been superseded by [RFC 3629](https://datatracker.ietf.org/doc/html/rfc3629) (2003).