This is just a feature request.
If you want to create / edit a UNIQUE/PRIMARY key index for a partitioned table, all columns have to be included. Otherwise a MySQL error is thrown.
Maybe this can be handled already in PMA as test before executing the statement.
Ticket moved from /p/phpmyadmin/bugs/4726/
Reference: http://dev.mysql.com/doc/refman/5.6/en/partitioning-limitations-partitioning-keys-unique-keys.html
I'm not sure whether this is worth the effort.