recent-conversations: Check for data change when processing message.

Add in a check to `recent_topics_data.process_message` so that we
know if any conversation data was updated and can rerender the
based on that information.

static/js/recent_topics_data.js

@@ -1,12 +1,15 @@
 import * as people from "./people";
 import {get_key_from_message} from "./recent_topics_util";
 
-export const topics = new Map(); // Key is stream-id:topic.
+export const topics = new Map();
+// For stream messages, key is stream-id:topic.
+// For pms, key is the user IDs to whom the message is being sent.
 
 export function process_message(msg) {
-    // Initialize topic and pm data
-    // Key for private message is the user id's
-    // to whom the message is begin sent.
+    // Return whether any conversation data is updated.
+    let conversation_data_updated = false;
+
+    // Initialize conversation data
     const key = get_key_from_message(msg);
     if (!topics.has(key)) {
         topics.set(key, {
@@ -14,9 +17,9 @@ export function process_message(msg) {
             participated: false,
             type: msg.type,
         });
+        conversation_data_updated = true;
     }
-    // Update topic data
-    const is_ours = people.is_my_user_id(msg.sender_id);
+    // Update conversation data
     const topic_data = topics.get(key);
     if (topic_data.last_msg_id < msg.id) {
         // NOTE: This also stores locally echoed msg_id which
@@ -24,13 +27,18 @@ export function process_message(msg) {
         // We store it now and reify it when response is available
         // from server.
         topic_data.last_msg_id = msg.id;
+        conversation_data_updated = true;
     }
     // TODO: Add backend support for participated topics.
     // Currently participated === recently participated
     // i.e. Only those topics are participated for which we have the user's
     // message fetched in the topic. Ideally we would want this to be attached
     // to topic info fetched from backend, which is currently not a thing.
-    topic_data.participated = is_ours || topic_data.participated;
+    if (!topic_data.participated && people.is_my_user_id(msg.sender_id)) {
+        topic_data.participated = true;
+        conversation_data_updated = true;
+    }
+    return conversation_data_updated;
 }
 
 function get_sorted_topics() {

static/js/recent_topics_ui.js

@@ -298,10 +298,17 @@ export function process_messages(messages) {
     // the UX can be bad if user wants to scroll down the list as
     // the UI will be returned to the beginning of the list on every
     // update.
+    let conversation_data_updated = false;
     if (messages.length > 0) {
         for (const msg of messages) {
-            process_message(msg);
+            if (process_message(msg)) {
+                conversation_data_updated = true;
+            }
         }
+    }
+
+    // Only rerender if conversation data actually changed.
+    if (conversation_data_updated) {
         complete_rerender();
     }
 }