recent: Sort PM recipients by their last sent message id.

Fixes zulip#23563

frontend_tests/node_tests/message_store.js

@@ -15,6 +15,7 @@ mock_esm("../../static/js/stream_topic_history", {
 
 mock_esm("../../static/js/recent_senders", {
     process_stream_message: noop,
+    process_private_message: noop,
 });
 
 set_global("document", "document-stub");

frontend_tests/node_tests/recent_senders.js

@@ -26,6 +26,9 @@ function make_stream_message({stream_id, topic, sender_id}) {
 mock_esm("../../static/js/message_store", {
     get: (message_id) => messages.get(message_id),
 });
+mock_esm("../../static/js/people", {
+    my_current_user_id: () => 1,
+});
 
 const rs = zrequire("recent_senders");
 zrequire("message_util.js");
@@ -310,3 +313,38 @@ test("process_stream_message", () => {
         true,
     );
 });
+
+test("process_pms", () => {
+    const sender1 = 1; // Current user id
+    const sender2 = 2;
+    const sender3 = 3;
+
+    const user_ids_string = "2,3,4";
+    rs.process_private_message({
+        to_user_ids: user_ids_string,
+        sender_id: sender2,
+        id: 1,
+    });
+    rs.process_private_message({
+        to_user_ids: user_ids_string,
+        sender_id: sender3,
+        id: 2,
+    });
+    rs.process_private_message({
+        to_user_ids: user_ids_string,
+        sender_id: sender1,
+        id: 3,
+    });
+
+    // Recent topics displays avatars in the opposite order to this since
+    // that was simpler to implement in HTML.
+    assert.deepEqual(rs.get_pm_recent_senders(user_ids_string), {
+        participants: [1, 3, 2],
+        non_participants: [4],
+    });
+    // PM doesn't exist.
+    assert.deepEqual(rs.get_pm_recent_senders("1000,2000"), {
+        participants: [],
+        non_participants: [],
+    });
+});

static/js/message_helper.js

@@ -65,6 +65,7 @@ export function process_new_message(message) {
 
             pm_conversations.process_message(message);
 
+            recent_senders.process_private_message(message);
             if (people.is_my_user_id(message.sender_id)) {
                 for (const recip of message.display_recipient) {
                     message_user_ids.add_user_id(recip.id);

static/js/recent_senders.js

@@ -2,6 +2,7 @@ import _ from "lodash";
 
 import {FoldDict} from "./fold_dict";
 import * as message_store from "./message_store";
+import * as people from "./people";
 
 // This class is only exported for unit testing purposes.
 // If we find reuse opportunities, we should just put it into
@@ -46,6 +47,9 @@ const stream_senders = new Map();
 // topic_senders[stream_id][topic_id][sender_id] = IdTracker
 const topic_senders = new Map();
 
+// pm_senders[user_ids_string][user_id] = IdTracker
+const pm_senders = new Map();
+
 export function clear_for_testing() {
     stream_senders.clear();
     topic_senders.clear();
@@ -211,3 +215,38 @@ export function get_topic_recent_senders(stream_id, topic) {
     }
     return recent_senders;
 }
+
+export function process_private_message({to_user_ids, sender_id, id}) {
+    const sender_dict = pm_senders.get(to_user_ids) || new Map();
+    const id_tracker = sender_dict.get(sender_id) || new IdTracker();
+    pm_senders.set(to_user_ids, sender_dict);
+    sender_dict.set(sender_id, id_tracker);
+    id_tracker.add(id);
+}
+
+export function get_pm_recent_senders(user_ids_string) {
+    const user_ids = user_ids_string.split(",").map((id) => Number.parseInt(id, 10));
+    const sender_dict = pm_senders.get(user_ids_string);
+    const pm_senders_info = {participants: [], non_participants: []};
+    if (!sender_dict) {
+        return pm_senders_info;
+    }
+
+    function compare_pm_user_ids_by_recency(user_id1, user_id2) {
+        const max_id1 = sender_dict.get(user_id1)?.max_id() || -1;
+        const max_id2 = sender_dict.get(user_id2)?.max_id() || -1;
+        return max_id2 - max_id1;
+    }
+
+    // Add current user to user_ids.
+    user_ids.push(people.my_current_user_id());
+    pm_senders_info.non_participants = user_ids.filter((user_id) => {
+        if (sender_dict.get(user_id)) {
+            pm_senders_info.participants.push(user_id);
+            return false;
+        }
+        return true;
+    });
+    pm_senders_info.participants.sort(compare_pm_user_ids_by_recency);
+    return pm_senders_info;
+}

static/js/recent_topics_ui.js

@@ -428,10 +428,6 @@ function format_conversation(conversation_data) {
         context.pm_url = last_msg.pm_with_url;
         context.is_group = last_msg.display_recipient.length > 2;
 
-        // Display in most recent sender first order
-        all_senders = last_msg.display_recipient;
-        senders = all_senders.slice(-MAX_AVATAR).map((sender) => sender.id);
-
         if (!context.is_group) {
             const user_id = Number.parseInt(last_msg.to_user_ids, 10);
             const user = people.get_by_user_id(user_id);
@@ -444,6 +440,17 @@ function format_conversation(conversation_data) {
             }
         }
 
+        // Since the css for displaying senders in reverse order is much simpler,
+        // we provide our handlebars with senders in opposite order.
+        // Display in most recent sender first order.
+        // To match the behavior for streams, we display the set of users who've actually
+        // participated, with the most recent participants first. It could make sense to
+        // display the other recipients on the PM conversation with different styling,
+        // but it's important to not destroy the information of "who's actually talked".
+        all_senders = recent_senders
+            .get_pm_recent_senders(context.user_ids_string)
+            .participants.reverse();
+        senders = all_senders.slice(-MAX_AVATAR);
         // Collect extra senders fullname for tooltip.
         extra_sender_ids = all_senders.slice(0, -MAX_AVATAR);
         displayed_other_senders = extra_sender_ids