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proposals/3825-fix-relation-fallbacks.md

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+# MSC3825: Obvious relation fallback location
+
+[MSC3440](https://github.com/matrix-org/matrix-spec-proposals/pull/3440) defines
+a way to use replies as a fallback for threads. For that it adds an
+`is_falling_back` key. Alternatively other relations could be marked as a
+fallback in the future if we get multiple relations or other advanced features.
+Here are 2 examples, decide which one marks the reply as being a fallback and
+which one marks the thread as being a fallback:
+
+```jsonc
+"m.relates_to": {
+    "rel_type": "m.thread",
+    "event_id": "ev1",
+    "is_falling_back": true,
+    "m.in_reply_to": {
+        "event_id": "last_event_id_in_thread",
+    }
+}
+```
+
+```jsonc
+"m.relates_to": {
+    "rel_type": "m.thread",
+    "event_id": "ev1",
+    "m.in_reply_to": {
+        "event_id": "last_event_id_in_thread",
+        "is_falling_back": true,
+    }
+}
+```
+
+You probably have read 3440 so you know that the first option is marking the
+reply in the thread relation as being a fallback. It is however not very
+logical. The second option is much clearer. It also allows generic handling of
+fallbacks in tools processing only the relations of the event like when handling
+[pushrules for relations](https://github.com/matrix-org/matrix-spec-proposals/pull/3664).
+
+## Proposal
+
+Move the `is_falling_back` key into the relation it applies to. This means for
+replies:
+
+```jsonc
+"m.in_reply_to": {
+    "event_id": "last_event_id_in_thread",
+    "is_falling_back": true,
+}
+```
+
+And for other relations:
+
+
+```jsonc
+"m.relates_to": {
+    "rel_type": "my.custom.relation",
+    "event_id": "ev1",
+    "is_falling_back": true,
+}
+```
+
+For the transition period clients might still accept the old location for
+threads until the ecosystem has been migrated.
+
+The goal is to merge this change immediately before threads land in the specification.
+
+## Potential issues
+
+This is a breaking change to an already merged feature. This means it will break
+events already sent into rooms and make them render incorrectly. This "should be
+fine" for several reasons:
+
+- The breakage means old events might show a reply relation when they should
+    not. This is annoying and not pretty, but not the end of the world.
+- Clients can mitigate by hardcoding a specific `origin_server_ts` to apply the
+    old rules to events. While it is strongly recommended to drop this at some
+    point for maintainability reasons, it should be fairly low overhead. Once
+    relations become editable, we could also fixup those events for other
+    clients.
+- Thread (as of writing) are not part of a released spec version. They are
+    marked as beta and users expect some things to not work optimally. As such
+    minor breakage should be acceptable. They will become part of the spec in
+    presumably end of Q3 2022. If we want to fix it, now is probably a better
+    time than later.
+
+## Alternatives
+
+1. Just accept that threads are weird and never make `is_falling_back` a generic
+   field. `is_falling_back` is a nice name and has otherwise clear semantics. I
+   would prefer to not take that route.
+2. Fix it later. Fixing this issue later is possible by introducing a new field
+   and deprecating the old one. The breakage would probably be greater.
+3. Special-case threads to have their field in that location. Not a pretty
+   solution and prevents multiple relations to ever fall back to threads without
+   introducing a different format.
+
+## Security considerations
+
+This introduces some ambiguities, so in theory it could be used to confuse
+users. Impact should be fairly limited.
+
+## Unstable prefix
+
+Until this MSC has finished FCP, implementations should use
+`im.nheko.msc3825.is_falling_back` as the fieldname instead.
+
+## Dependencies
+
+This MSC depends on threads (MSC3440), which is already merged.
+
+It improves functionality in preparation of
+- Pushrules for relations,
+    [MSC3664](https://github.com/matrix-org/matrix-spec-proposals/pull/3664)
+- Multiple relations or other MSCs building on top of relations that could
+    benefit from fallback semantics like
+    [MSC3051](https://github.com/matrix-org/matrix-spec-proposals/pull/3051)