On shared folders received, both Shared and Public, with read-only ACL by selecting multiple messages, it is not possible to change the Seen / Unseen flag.
Changing flags on a single message works.
Steps to reproduce
- share a folder to user user1 with read-only ACL
- copy some e-mails in unseen status to this folder
- log in with user1 and open the shared folder
- select multiple messages
- right click -> Mark Seen
- the messages are all marked as Seen
- messages remain marked as Unseen and no actions are reported in the imap log.