2153-reaction-push-rule.md

Add a default push rule to ignore m.reaction events

Reactions are considered "metadata" that annotates an existing event and thus they should not by default trigger notifications. (See MSC1849 for the reaction event format.)

This is especially important for rooms that may be set with a room-specific rule to notify, as they will trigger notifications for every reaction and it's tricky to clear them as well. See element-hq/element-web#10208 for details.

Proposal

A new default override rule is to be added that ignores reaction events:

{
    "rule_id": ".m.rule.reaction",
    "default": true,
    "enabled": true,
    "conditions": [
        {
            "kind": "event_match",
            "key": "type",
            "pattern": "m.reaction"
        }
    ],
    "actions": [
        "dont_notify"
    ]
}

Tradeoffs

We could instead allow notifications for reactions in some cases (the current state) but then modify each client with complex heuristics to clear them by advancing the read receipt. This would be more involved than for regular messages because reaction events may target any event in the timeline, so there's no guarantee that a new reaction event targets something that's currently displayed to the user. By going with the push rule above, client developers won't have to deal with this issue.