diff --git a/frontend_tests/node_tests/message_fetch.js b/frontend_tests/node_tests/message_fetch.js
index fa173dc329..967d0fe8bf 100644
--- a/frontend_tests/node_tests/message_fetch.js
+++ b/frontend_tests/node_tests/message_fetch.js
@@ -7,6 +7,7 @@ const _ = require("lodash");
 const {mock_esm, set_global, zrequire} = require("../zjsunit/namespace");
 const {run_test} = require("../zjsunit/test");
 const $ = require("../zjsunit/zjquery");
+const {page_params} = require("../zjsunit/zpage_params");
 
 set_global("document", "document-stub");
 
@@ -267,6 +268,9 @@ run_test("initialize", () => {
     reset_lists();
 
     let home_loaded = false;
+    page_params.unread_msgs = {
+        old_unreads_missing: false,
+    };
 
     function home_view_loaded() {
         home_loaded = true;
@@ -351,6 +355,9 @@ run_test("loading_newer", () => {
 
     (function test_narrow() {
         const msg_list = simulate_narrow();
+        page_params.unread_msgs = {
+            old_unreads_missing: true,
+        };
 
         const data = {
             req: {
diff --git a/frontend_tests/node_tests/unread.js b/frontend_tests/node_tests/unread.js
index 7e855fc558..a119ff638c 100644
--- a/frontend_tests/node_tests/unread.js
+++ b/frontend_tests/node_tests/unread.js
@@ -514,7 +514,7 @@ test("mentions", () => {
     };
 
     const muted_direct_mention_message = {
-        id: 17,
+        id: 18,
         type: "stream",
         stream_id: muted_stream_id,
         topic: "lunch",
@@ -542,6 +542,7 @@ test("mentions", () => {
         mention_me_message.id,
         mention_all_message.id,
         muted_mention_all_message.id,
+        muted_direct_mention_message.id,
     ]);
     test_notifiable_count(counts.home_unread_messages, 3);
 
diff --git a/static/js/message_events.js b/static/js/message_events.js
index a6c4fc785c..5083b8896c 100644
--- a/static/js/message_events.js
+++ b/static/js/message_events.js
@@ -116,7 +116,7 @@ function maybe_add_narrowed_messages(messages, msg_list, callback, attempt = 1)
 export function insert_new_messages(messages, sent_by_this_client) {
     messages = messages.map((message) => message_helper.process_new_message(message));
 
-    unread.process_loaded_messages(messages);
+    const any_untracked_unread_messages = unread.process_loaded_messages(messages, false);
     huddle_data.process_loaded_messages(messages);
 
     // all_messages_data is the data that we use to populate
@@ -153,7 +153,9 @@ export function insert_new_messages(messages, sent_by_this_client) {
         notifications.notify_local_mixes(messages, need_user_to_scroll);
     }
 
-    unread_ui.update_unread_counts();
+    if (any_untracked_unread_messages) {
+        unread_ui.update_unread_counts();
+    }
     resize.resize_page_components();
 
     unread_ops.process_visible();
diff --git a/static/js/message_fetch.js b/static/js/message_fetch.js
index 8cee10b510..252763b780 100644
--- a/static/js/message_fetch.js
+++ b/static/js/message_fetch.js
@@ -49,16 +49,9 @@ function process_result(data, opts) {
 
     messages = messages.map((message) => message_helper.process_new_message(message));
 
-    // In case any of the newly fetched messages are new, add them to
-    // our unread data structures.  It's important that this run even
-    // when fetching in a narrow, since we might return unread
-    // messages that aren't in the home view data set (e.g. on a muted
-    // stream).
-    //
-    // BUG: This code path calls pm_list.update_private_messages, even
-    // if there were no private messages (or even no new messages at
-    // all) in data.messages, which is a waste of resources.
-    message_util.do_unread_count_updates(messages);
+    // In some rare situations, we expect to discover new unread
+    // messages not tracked in unread.js during this fetching process.
+    message_util.do_unread_count_updates(messages, true);
 
     // If we're loading more messages into the home view, save them to
     // the all_messages_data as well, as the message_lists.home is
diff --git a/static/js/message_util.js b/static/js/message_util.js
index 56e085b60a..3545ac952b 100644
--- a/static/js/message_util.js
+++ b/static/js/message_util.js
@@ -7,10 +7,15 @@ import * as resize from "./resize";
 import * as unread from "./unread";
 import * as unread_ui from "./unread_ui";
 
-export function do_unread_count_updates(messages) {
-    unread.process_loaded_messages(messages);
-    unread_ui.update_unread_counts();
-    resize.resize_page_components();
+export function do_unread_count_updates(messages, expect_no_new_unreads = false) {
+    const any_new_unreads = unread.process_loaded_messages(messages, expect_no_new_unreads);
+
+    if (any_new_unreads) {
+        // The following operations are expensive, and thus should
+        // only happen if we found any unread messages justifying it.
+        unread_ui.update_unread_counts();
+        resize.resize_page_components();
+    }
 }
 
 export function add_messages(messages, msg_list, opts) {
diff --git a/static/js/unread.js b/static/js/unread.js
index c8c5ae5659..2b43f2b465 100644
--- a/static/js/unread.js
+++ b/static/js/unread.js
@@ -1,3 +1,4 @@
+import * as blueslip from "./blueslip";
 import {FoldDict} from "./fold_dict";
 import * as message_store from "./message_store";
 import {page_params} from "./page_params";
@@ -537,9 +538,34 @@ export function update_unread_topics(msg, event) {
     });
 }
 
-export function process_loaded_messages(messages) {
+export function process_loaded_messages(messages, expect_no_new_unreads = false) {
+    // Process a set of messages that we have full copies of from the
+    // server for whether any are unread but not tracked as such by
+    // our data structures. This can occur due to old_unreads_missing,
+    // changes in muting configuration, innocent races, or potentially bugs.
+    //
+    // Returns whether there were any new unread messages; in that
+    // case, the caller will need to trigger a rerender of UI
+    // displaying unread counts.
+
+    let any_untracked_unread_messages = false;
     for (const message of messages) {
         if (message.unread) {
+            if (unread_messages.has(message.id)) {
+                // If we're already tracking this message as unread, there's nothing to do.
+                continue;
+            }
+
+            if (expect_no_new_unreads && !page_params.unread_msgs.old_unreads_missing) {
+                // This may happen due to races, where someone narrows
+                // to a view and the message_fetch request returns
+                // before server_events system delivers the message to
+                // the client.
+                //
+                // For now, log it as a blueslip error so we can learn its prevalence.
+                blueslip.error("New unread discovered in process_loaded_messages.");
+            }
+
             const user_ids_string =
                 message.type === "private" ? people.pm_reply_user_string(message) : undefined;
 
@@ -553,8 +579,11 @@ export function process_loaded_messages(messages) {
                 unread: true,
                 user_ids_string,
             });
+            any_untracked_unread_messages = true;
         }
     }
+
+    return any_untracked_unread_messages;
 }
 
 export function process_unread_message(message) {