When running MySQL/MariaDB with charset utf8mb4, a character is 4 bytes. Hence a key created on a VARCHAR(255) takes 255 characters * 4 bytes/characters = 1020 bytes. However Innodb defaults to a maximum of 767 bytes for an index.
The extension fails to install on Debian Stretch which uses utf8mb4 as the default charset:
Query: CREATE UNIQUE INDEX Comments_Vote_user_id_index ON `Comments_Vote` (Comment_Vote_ID,Comment_Vote_Username) Function: Wikimedia\Rdbms\Database::sourceFile( /workspace/src/extensions/Comments/includes/../sql/comments.sql ) Error: 1071 Specified key was too long; max key length is 767 bytes (localhost:/tmp/quibble-mysql-wp_s7n4r/socket)