[Story] Manually mark a room as unread

[VolkerJunginger]

Description

As a user that has read the messages in a room but does not have the time to react I want to manually mark a room as unread in the room-list so that I can easily identify room with open todos.

Product requirements: https://docs.google.com/document/d/15Rk9ERkQNOM9yQubbWEh-XpMzRL2iiN5HoSe_SybBbo/edit

Acceptance criteria

N.B. Is has been decided that the old implementation of this on Legacy Element iOS will be removed to avoid conflicts between clients.

• expose "mark as (un)read" options on individual rooms on the room list
• once marked as unread the room should show a green badge. If new events come in, the room should continue showing the green badge and optionally a green mention badge, if a mention ocurred
• rooms marked as unread should show a green badge even if their notification settings are set to mentions&keywords or muted. After being marked as unread or being rendered a room should go back to showing the default colors for those options
• marking the room as read from the room list should remove all badges and send read receipts, marking everything as read
• when entering a room, the manual unread flag should be unset but read receipts should not be sent. The timeline should handle that based on what the current position is and what is actually visible (especially after the fully read marker will start informing positions)

Leads

• Tech: @stefanceriu
• Design:

Size estimate

M

Dependencies

• Maybe MSC https://github.com/matrix-org/matrix-spec-proposals/pull/2867 if we decide to use it.

Out of scope

• Nothing

Open questions

### Questions
- [x] Does https://github.com/matrix-org/matrix-spec-proposals/pull/2867 fit our need? It is used by several matrix clients.
- [x] Is it ok to use this MSC? --> YES

Subtasks

### Web
- [ ] https://github.com/element-hq/wat-internal/issues/136

### Android
- [ ] https://github.com/element-hq/element-x-android/issues/2261

### iOS
- [ ] https://github.com/element-hq/element-x-ios/issues/2360

### Rust
- [ ] https://github.com/matrix-org/matrix-rust-sdk/issues/3037
- [ ] https://github.com/matrix-org/matrix-rust-sdk/pull/3111

### Other

Sign-off

Web

• [ ] Design sign-off on completion
• [ ] QA sign-off on completion
• [ ] Product sign-off on completion

Android

• [ ] Design sign-off on completion
• [ ] QA sign-off on completion
• [ ] Product sign-off on completion

iOS

• [ ] Design sign-off on completion
• [ ] QA sign-off on completion
• [ ] Product sign-off on completion

[manuroe]

The MSC is currently scheduled for the next version 1.10 which should be end of Feb.

[daniellekirkwood]

This is still open on Web though all the dev work is done, just not the launch :)

here's the posthog: https://posthog.hss.element.io/insights/dOHeC0O2