MSC3765: Rich text in room topics

proposals/3765-rich-room-topics.md

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+# MSC3765: Rich text in room topics
+
+## Problem
+
+Topics are a central piece of room meta data and usually made easily
+accessible to room members in clients. As a result, room administrators
+often extend the use of topics to collect helpful peripheral information
+that is related to the room’s purpose. Most commonly these are links to
+external resources. At the moment, topics are limited to [plain text]
+which, depending on the number and length of URLs and other content,
+easily gets inconvenient to consume and calls for richer text formatting
+options.
+
+## Proposal
+
+Drawing from extensible events as described in [MSC1767], `m.room.topic`
+is prohibited in room versions that support extensible events and replaced
+with a new `m.topic` event type. The latter contains a new content block
+`m.topic` which wraps an `m.text` content block that allows representing
+the room topic in different mime types.
+
+``` json5
+{
+  "type": "m.topic",
+  "state_key": "",
+  "content": {
+    "m.topic": {
+      "m.text": [{
+          "body": "All about **pizza** | [Recipes](https://recipes.pizza.net)"
+      }, {
+          "mimetype": "text/html",
+          "body": "All about <b>pizza</b> | <a href=\"https://recipes.pizza.net\">Recipes</a>"
+      }]
+    }
+  },
+  ...
+}
+```
+
+Details of how `m.text` works may be found in [MSC1767] and are not
+repeated here.
+
+The wrapping `m.topic` content block is similar to `m.caption` for file
+uploads as defined in [MSC3551]. It avoids clients accidentally rendering
+the topic state event as a room message.
+
+In order to prevent formatting abuse in room topics, clients are
+encouraged to limit the length of topics to at most two lines. Additionally,
+clients should ignore things like headings and enumerations (or format them
+as regular text). A future MSC may introduce a mechanism to capture extended
+multiline details that are not suitable for room topics in a separate field
+or event type.
+
+A change to `/_matrix/client/v3/createRoom` is not necessary. The
+endpoint has a plain text `topic` parameter but also allows to specify a
+full `m.topic` event in `initial_state`.
+
+Room topics also occur as part of the `PublicRoomsChunk` object in the
+responses of `/_matrix/client/v3/publicRooms` and
+`/_matrix/client/v1/rooms/{roomId}/hierarchy`. The topic can be kept
+plain text here because this data should commonly only be displayed to
+users that are *not* a member of the room yet. These users will not have
+the same need for rich room topics as users who are inside the room. If
+no plain text topic exists, home servers should return an empty topic
+string from these end points. Since this will inevitably lead to bad UX,
+client implementations are encouraged to always include a plain text
+variant when sending `m.topic` events.
+
+## Transition
+
+As this MSC replaces `m.room.topic` for an extensible alternative,
+clients and servers are expected to treat `m.room.topic` as invalid in
+extensible event-supporting room versions.
+
+## Potential issues
+
+None.
+
+## Alternatives
+
+The combination of `format` and `formatted_body` currently utilised to
+enable HTML in `m.room.message` events could be generalised to
+`m.topic` events. However, this would only allow for a single
+format in addition to plain text and is a weaker form of reuse than
+described in the introductory section of [MSC1767].
+
+## Security considerations
+
+Allowing HTML in room topics is subject to the same security
+considerations that apply to HTML in room messages.
+
+## Unstable prefix
+
+While this MSC is not considered stable, `m.topic` should be referred to
+as `org.matrix.msc3765.topic`.
+
+## Dependencies
+
+- [MSC1767]
+
+  [plain text]: https://spec.matrix.org/v1.2/client-server-api/#mroomtopic
+  [MSC1767]: https://github.com/matrix-org/matrix-spec-proposals/pull/1767
+  [MSC3551]: https://github.com/matrix-org/matrix-spec-proposals/pull/3551
+  [`/rooms/{roomId}/upgrade`]: https://spec.matrix.org/v1.5/client-server-api/#post_matrixclientv3roomsroomidupgrade