Implement MSC4142: Remove unintentional intentional mentions in replies

Signed-off-by: Tulir Asokan <tulir@maunium.net>

Author: tulir

src/components/views/rooms/SendMessageComposer.tsx

@@ -100,12 +100,6 @@ export function attachMentions(
     // event + any mentioned users in that event.
     if (replyToEvent) {
         userMentions.add(replyToEvent.sender!.userId);
-        // TODO What do we do if the reply event *doeesn't* have this property?
-        // Try to fish out replies from the contents?
-        const userIds = replyToEvent.getContent()["m.mentions"]?.user_ids;
-        if (Array.isArray(userIds)) {
-            userIds.forEach((userId) => userMentions.add(userId));
-        }
     }
 
     // If user provided content is available, check to see if any users are mentioned.

test/components/views/rooms/EditMessageComposer-test.tsx

@@ -432,8 +432,6 @@ describe("<EditMessageComposer/>", () => {
                     user_ids: [
                         // sender of event we replied to
                         originalEvent.getSender()!,
-                        // mentions from this event
-                        "@bob:server.org",
                     ],
                 },
             },

test/components/views/rooms/SendMessageComposer-test.tsx

@@ -196,7 +196,7 @@ describe("<SendMessageComposer/>", () => {
                 "m.mentions": { user_ids: ["@bob:test"] },
             });
 
-            // It also adds any other mentioned users, but removes yourself.
+            // It no longer adds any other mentioned users
             replyToEvent = mkEvent({
                 type: "m.room.message",
                 user: "@bob:test",
@@ -207,7 +207,7 @@ describe("<SendMessageComposer/>", () => {
             content = {};
             attachMentions("@alice:test", content, model, replyToEvent);
             expect(content).toEqual({
-                "m.mentions": { user_ids: ["@bob:test", "@charlie:test"] },
+                "m.mentions": { user_ids: ["@bob:test"] },
             });
         });