'Undo log record is too big.' error occurring in very narrow range of string lengths

A user is seeing an 'Undo log record is too big.' error that only occurs if they update 2 mediumtext fields at once, and if the length of the existing values in each those fields happen to be between 3962 and 4030 characters long.

Some other interesting notes about this:

• The table definition has ROW_FORMAT=COMPACT, so it uses the Antelope format.

• The only indexes on the two mediumtext fields have prefix lengths of 255.

• The problem is repeatable on MySQL 5.5 and 5.7, and MariaDB 10.1 and 10.2, so I don't think it has been fixed in any released version of MySQL or MariaDB.

I have attached an SQL file that can be used to reproduce this issue.

Upstream bug report:

https://bugs.mysql.com/bug.php?id=88150