events: Don't add reply fallback in thread fallback

crates/ruma-common/src/events/room/message.rs

@@ -239,14 +239,17 @@ impl RoomMessageEventContent {
     /// thread is created. If it doesn't, a new thread with `previous_message` as the root is
     /// created.
     ///
-    /// If `message` is a text or notice message, it is modified to include the rich reply fallback.
+    /// If `message` is a text or notice message, and this is a reply in the thread, it is modified
+    /// to include the rich reply fallback.
     #[cfg(feature = "unstable-msc3440")]
     pub fn for_thread(
         message: MessageType,
         previous_message: &OriginalRoomMessageEvent,
         is_reply: ReplyInThread,
     ) -> Self {
-        let msgtype = match message {
+        let msgtype = if is_reply == ReplyInThread::Yes {
+            // If this is a real reply, add the rich reply fallback.
+            match message {
                 MessageType::Text(TextMessageEventContent { body, formatted, .. }) => {
                     let (body, html_body) = reply::plain_and_formatted_reply_body(
                         body,
@@ -266,6 +269,9 @@ impl RoomMessageEventContent {
                     MessageType::Notice(NoticeMessageEventContent::html(body, html_body))
                 }
                 _ => message,
+            }
+        } else {
+            message
         };
 
         let thread_root = if let Some(Relation::Thread(Thread { event_id, .. })) =