Allow regular members to reply in threads in read-only rooms #42688

Problem

Members who want to discuss an announcement have to take the conversation somewhere else, like a different room or a DM. That keeps the main room clean, but it also splits the discussion.

Read-only rooms block regular members from posting, and that includes replies in threads. Members have no way to respond to a message in a thread. Admins and users with post-readonly can do so.

Use case

An announcements channel where admins post updates and members want to ask questions or respond. Right now the choices are:

• Let members post in the main room, which defeats read-only.
• Keep members silent and send discussion to another room or DM.
• Use a discussion, which requires separate setup and isn't a reply to the announcement.

Proposed behavior

When a room is read-only, regular members can reply in threads on existing messages, but can't post new messages in the main room. Possible ways to configure it:

1. A room setting, such as "Allow thread replies when read-only", off by default.
2. A permission, such as post-thread-readonly, granted to the roles that should be able to reply.

Either way, the main room should stay read-only for members, and thread replies should follow the existing thread rules.

Open questions

• Should this apply to broadcast rooms too, or only to read-only rooms?
• Should reactions in read-only rooms (reactWhenReadOnly) and thread replies share one setting, or be separate?
• How should this interact with federated rooms?