MSC4356: Recently used emoji

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

proposals/4356-recent-emoji.md

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+# MSC4356: Recently used emoji
+
+Like other chat platforms, Matrix supports emoji as a way to visually express ideas or emotions. In
+practice, most people use a limited set of emoji only. Since emoji are commonly used as a quick way
+to react to something, it is desirable for clients to offer users shortcuts to their favorite emoji.
+Some emoji picker libraries support this feature by locally tracking emoji usage. This doesn't work
+well in a multi-device environment, however, because such history cannot easily be shared between
+clients.
+
+This proposal introduces a way for clients to maintain a shared storage of recently used emoji to
+enable emoji suggestions across clients.
+
+## Proposal
+
+A new global account data event `m.recent_emoji` is introduced. In `content`, it contains a single
+property `recent_emoji` that is an array where each element is itself an array. The first element in
+this nested array is the emoji, the second element is a counter for how often it was used. The outer
+`recent_emoji` array is ordered descendingly by last usage time.
+
+``` json5
+{
+  "type": "m.recent_emoji",
+  "content": {
+    "recent_emoji": [
+      [ "😅",  7 ], // Most recently used, 7 times overall
+      [ "👍", 84 ], // Second most recently used, 84 times overall
+      ...
+  }
+}
+```
+
+When an emoji is used in a message or an annotation, the sending client moves (or adds) it to the
+beginning of the `recent_emoji` array and increments (or initializes) its counter.
+
+As new emoji are being used, clients SHOULD limit the length of the `recent_emoji` array by dropping
+elements from the end. A RECOMMENDED maximum length is 100 emoji.
+
+Clients MAY freely customise the logic for generating recommendations from the stored emoji. As an
+example, they could select the 24 first (= most recently used) emoji and stably sort them by their
+counters (so that more recently used emoji are ordered first on ties).
+
+## Potential issues
+
+Clients could choose wildly different ways to generate recommendations from the shared storage
+leading to significantly different UX across clients.
+
+## Alternatives
+
+Further metadata such as the concrete access time or the room could be tracked together with emoji.
+It is unclear, however, if this would lead to materially better suggestions, however.
+
+## Security considerations
+
+This proposal doesn't mandate encrypting the `m.recent_emoji` account data event. Since emoji are
+most commonly used in annotations which are not encrypted, servers could already track and abuse
+this information today, however.
+
+## Unstable prefix
+
+While this MSC is not considered stable, `m.recent_emoji` should be referred to as
+`io.element.recent_emoji`.
+
+## Dependencies
+
+None.