Handle message edits.

src/components/views/rooms/EditMessageComposer.tsx

@@ -16,7 +16,7 @@ limitations under the License.
 
 import React, { createRef, KeyboardEvent } from "react";
 import classNames from "classnames";
-import { EventStatus, IContent, MatrixEvent } from "matrix-js-sdk/src/models/event";
+import { EventStatus, IContent, IMentions, MatrixEvent } from "matrix-js-sdk/src/models/event";
 import { MsgType } from "matrix-js-sdk/src/@types/event";
 import { Room } from "matrix-js-sdk/src/models/room";
 import { logger } from "matrix-js-sdk/src/logger";
@@ -29,7 +29,7 @@ import { getCaretOffsetAndText } from "../../../editor/dom";
 import { htmlSerializeIfNeeded, textSerialize, containsEmote, stripEmoteCommand } from "../../../editor/serialize";
 import { findEditableEvent } from "../../../utils/EventUtils";
 import { parseEvent } from "../../../editor/deserialize";
-import { CommandPartCreator, Part, PartCreator, SerializedPart } from "../../../editor/parts";
+import { CommandPartCreator, Part, PartCreator, SerializedPart, Type } from "../../../editor/parts";
 import EditorStateTransfer from "../../../utils/EditorStateTransfer";
 import BasicMessageComposer, { REGEX_EMOTICON } from "./BasicMessageComposer";
 import { CommandCategories } from "../../../SlashCommands";
@@ -47,6 +47,7 @@ import { getSlashCommand, isSlashCommand, runSlashCommand, shouldSendAnyway } fr
 import { KeyBindingAction } from "../../../accessibility/KeyboardShortcuts";
 import { PosthogAnalytics } from "../../../PosthogAnalytics";
 import { editorRoomKey, editorStateKey } from "../../../Editing";
+import { attachMentions } from "./SendMessageComposer";
 
 function getHtmlReplyFallback(mxEvent: MatrixEvent): string {
     const html = mxEvent.getContent().formatted_body;
@@ -103,12 +104,44 @@ function createEditContent(model: EditorModel, editedEvent: MatrixEvent): IConte
         contentBody.formatted_body = `${htmlPrefix} * ${formattedBody}`;
     }
 
+    // Build the mentions property for the *new* content (as if there was no edit).
+    //
+    // TODO If this is a reply we need to include all the users from it.
+    attachMentions(editedEvent.sender.userId, newContent, model, undefined);
+
+    // Attach the *differential* mentions of any *newly* mentioned users / room.
+    const mentions: IMentions = {};
+    // TODO What if the edit doesn't have the mentions property?
+    const prevMentions = editedEvent.getContent()["org.matrix.msc3952.mentions"];
+    let prevUserMentions = prevMentions?.user_ids;
+    if (!Array.isArray(prevUserMentions)) {
+        prevUserMentions = [];
+    }
+    const newUserMentions = new Set<string>();
+    for (const part of model.parts) {
+        // Only include the user if they were not previously mentioned and a room
+        // mention if the previous version did not include one.
+        if (part.type === Type.UserPill && !prevUserMentions.includes(part.resourceId)) {
+            newUserMentions.add(part.resourceId);
+        } else if (part.type === Type.AtRoomPill && !prevMentions?.room) {
+            mentions.room = true;
+        }
+    }
+
+    // Ensure the *current* user isn't listed in the mentioned users.
+    newUserMentions.delete(editedEvent.sender.userId);
+
+    if (newUserMentions.size) {
+        mentions.user_ids = [...newUserMentions];
+    }
+
     const relation = {
         "m.new_content": newContent,
         "m.relates_to": {
             rel_type: "m.replace",
             event_id: editedEvent.getId(),
         },
+        "org.matrix.msc3952.mentions": mentions,
     };
 
     return Object.assign(relation, contentBody);