MySQL 5.7 (which was merged to MariaDB 10.2.2) implements an instantaneous change of the maximum length of a VARCHAR column when the length is increasing and not crossing the 255-byte boundary.
Alas, when the VARCHAR column was indexed, the indexes would be dropped and added. This should be avoided.
MySQL 5.7.23 addressed this by changing the storage engine API. We would prefer a less intrusive change. InnoDB should be able to ‘optimize away’ pairs of DROP INDEX and ADD INDEX operations. (While doing that, InnoDB in MariaDB should support the renaming of indexes as well, without introducing the RENAME INDEX syntax from MySQL 5.7.)