Mix of aggregate window functions and GROUP BY produces confusing results

drop table if exists empsalary;

create table empsalary (depname varchar(32), empno smallint primary key, salary int);

insert into empsalary values  ('develop',1,5000),('develop',2,4000),('sales',3,'6000'),('sales',4,5000);

SELECT depname, empno, salary, avg(salary) OVER (PARTITION BY depname) FROM empsalary group by depname;

+---------+-------+--------+-----------------------------------------+

| depname | empno | salary | avg(salary) OVER (PARTITION BY depname) |

+---------+-------+--------+-----------------------------------------+

| develop |     1 |   5000 |                               5000.0000 |

| sales   |     3 |   6000 |                               6000.0000 |

+---------+-------+--------+-----------------------------------------+

2 rows in set (0.00 sec)