When a column is not a part of GROUP BY expression / an argument of aggregate function and ONLY_FULL_GROUP_BY server SQL mode is enabled then the error 1055 must be generated.
But when such non-aggregated column is an argument of the window function then ONLY_FULL_GROUP_BY server SQL mode is ignored and the query is executed w/o an error.

Sample data:

CREATE TABLE test (id INT, x INT);

INSERT INTO test VALUES (1,1), (1,2), (2,3), (2,4);

Query:

SELECT id, SUM(x) sum_x, SUM(x) OVER (PARTITION BY id) wsum_x

FROM test

GROUP BY id;

The error "Error 1055: Expression #3 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'fiddle.test.x' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by" or similar must be reproted - but the query is executed successfully.

Tested at MariaDB 10.3 and newer.