Allow read markers to go backwards

[nunoperalta]

Suggestion
In order for front-ends to be able to implement "Mark as Unread from this message" (like it's possible in Skype), Matrix needs to allow the read marker to go backwards.

See issue in Element: element-hq/element-meta#347

See this comment (and the following one): element-hq/element-meta#347 (comment)

Issue raised for Element for this specific feature: element-hq/element-meta#2465

Thank you!

[turt2live]

My current opinion is we should separate read receipts from notifications instead of allowing them go backwards. For reasons discussed on MSC2867, going backwards is way more complicated.

[alphapapa]

FWIW, Ement.el allows the user to set the read markers in a room to any event (what the server or other clients do with that, notwithstanding).

IMHO, the ability to mark a room as "unread from here" is important functionality that is missing in Matrix (officially, anyway).

As well, while I understand the motivation for #1895 and why it has been moved forward, I'm concerned that it overlaps with this proposed functionality in a way that will ultimately make supporting this proposed functionality more complicated than necessary.

IOW, how would it make sense to both a) mark a room as unread, and b) mark a room as "unread from here"? ISTM that, rather than marking a room as simply "unread," such marking should include an optional event ID, so that a room could be marked as either "simply unread" (without regard to a specific event) or "unread from here" (with regard to a specific event).

But if these features are implemented separately, imagine how much more complicated the logic will have to be on clients: checking multiple per-room data structures, reconciling potentially conflicting values (e.g. a room marked as "unread from here" but not "unread"), or other combinations which could result from using clients with varying Matrix version support.

I encourage the Matrix developers to consolidate these ideas into a single spec proposal that will address them both in a unified, simple way.