Add typing extension

proposals/3961-sliding-sync-typing.md

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+# MSC3961: Sliding Sync Extension: Typing Notifications
+
+This MSC is an extension to [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575)
+which includes support for typing notifications in the `/sync` response.
+
+## Proposal
+
+MSC3575 does not include support for typing notifications. This extension will add support for it.
+
+The prosposal is to introduce a new extension called `typing` with the following request parameters:
+```js
+{
+    "enabled": true // sticky
+}
+```
+If `enabled` is `true`, then the sliding sync response MAY include the following response fields in
+the `typing` extension response:
+```json
+{
+    "rooms": {
+        "!foo:bar": {
+            m.typing EDU
+        },
+        "!foo2:bar" {
+            m.typing EDU
+        }
+    }
+}
+```
+
+The `m.typing` event in this response is the same event that would appear in the array
+`rooms.join["!foo:bar"].ephemeral.events` under `/sync`. 
+
+On an initial sync, typing notifications MUST only be sent for rooms returned in the sliding sync response.
+When live streaming, typing notifications MUST be sent as the server receives typing updates. Rooms which
+initially appear (`initial: true`) due to direct subscriptions or rooms moving into the sliding window MUST
+have typing notification data added _if_ there are typing users, else it can be omitted from the typing response.
+
+## Potential issues
+
+No issues identified.
+
+## Alternatives
+
+This extension could be bundled into the core MSC3575, but this would force all clients to receive this
+data. In practice clients can function extremely well without the need for typing notifications, so forcing all
+clients to receive this data feels like the wrong design.
+
+## Security considerations
+
+No additional security considerations beyond what the current `/sync` implementation provides.
+
+## Unstable prefix
+
+No unstable prefix as Sliding Sync is still in review. To enable this extension, just add this to
+your request JSON:
+```js
+{
+    "extensions": {
+        "typing": {
+            "enabled": true
+        }
+    }
+}
+```
+
+## Dependencies
+
+This MSC builds on MSC3575, which at the time of writing has not yet been accepted into the spec.