email-notifs: Use bracketed prefix to indicate a resolved topic.

Some email clients (notably, Gmail Web) support automatically threading
emails together if recipients and subjects match[1]. Manual testing
indicated that prefixing a subject with "[bracketed content]" does not
break this threading behavior, but the added checkmark in a resolved
topic's title does. Before sending an email notification, determine
whether the topic is resolved, and pass this information to the Jinja
template to properly format a threadable email subject.

Fixes: #22538

[1]: https://support.google.com/mail/answer/5900

templates/zerver/emails/missed_message.subject.txt

@@ -1,7 +1,17 @@
 {% if show_message_content %}
     {% if group_pm %} {% trans %}Group PMs with {{ huddle_display_name }}{% endtrans %}
     {% elif private_message %} {% trans %}PMs with {{ sender_str }}{% endtrans %}
-    {% elif stream_email_notify or mention %} #{{ stream_name }} > {{ topic_name }}
+    {% elif stream_email_notify or mention %}
+        {#
+        Some email clients, like Gmail Web (as of 2022), will auto-thread
+        emails that share a subject and recipients, but will disregard
+        [Bracketed Prefixes] in the style of old-school mailing lists. We take
+        advantage of this to retain thread continuity in email notifications
+        even after a topic is resolved.
+        #}
+        {% if topic_resolved %}{% trans %}[resolved] #{{ stream_name }} > {{ topic_name }}{% endtrans %}
+        {% else %}#{{ stream_name }} > {{ topic_name }}
+        {% endif %}
     {% endif %}
 {% else %}
     {% trans %}New messages{% endtrans %}

zerver/lib/email_notifications.py

@@ -27,6 +27,7 @@ from zerver.lib.notification_data import get_mentioned_user_group_name
 from zerver.lib.queue import queue_json_publish
 from zerver.lib.send_email import FromAddress, send_future_email
 from zerver.lib.soft_deactivation import soft_reactivate_if_personal_notification
+from zerver.lib.topic import get_topic_resolution_and_bare_name
 from zerver.lib.types import DisplayRecipientT
 from zerver.lib.url_encoding import (
     huddle_narrow_url,
@@ -481,10 +482,11 @@ def do_send_missedmessage_events_reply_in_zulip(
             )
         message = missed_messages[0]["message"]
         stream = Stream.objects.only("id", "name").get(id=message.recipient.type_id)
-        topic_name = message.topic_name()
+        topic_resolved, topic_name = get_topic_resolution_and_bare_name(message.topic_name())
         context.update(
             stream_name=stream.name,
             topic_name=topic_name,
+            topic_resolved=topic_resolved,
         )
     else:
         raise AssertionError("Invalid messages!")

zerver/lib/topic.py

@@ -270,3 +270,17 @@ def get_topic_history_for_stream(
     cursor.close()
 
     return generate_topic_history_from_db_rows(rows)
+
+
+def get_topic_resolution_and_bare_name(stored_name: str) -> Tuple[bool, str]:
+    """
+    Resolved topics are denoted only by a title change, not by a boolean toggle in a database column. This
+    method inspects the topic name and returns a tuple of:
+
+    - Whether the topic has been resolved
+    - The topic name with the resolution prefix, if present in stored_name, removed
+    """
+    if stored_name.startswith(RESOLVED_TOPIC_PREFIX):
+        return (True, stored_name[len(RESOLVED_TOPIC_PREFIX) :])
+
+    return (False, stored_name)

zerver/tests/test_email_notifications.py

@@ -1053,6 +1053,13 @@ class TestMissedMessages(ZulipTestCase):
     def test_extra_context_in_missed_stream_messages_email_notify(self) -> None:
         self._extra_context_in_missed_stream_messages_email_notify(False)
 
+    @override_settings(SEND_MISSED_MESSAGE_EMAILS_AS_USER=True)
+    def test_resolved_topic_missed_stream_messages_thread_friendly_as_user(self) -> None:
+        self._resolved_topic_missed_stream_messages_thread_friendly(True)
+
+    def test_resolved_topic_missed_stream_messages_thread_friendly(self) -> None:
+        self._resolved_topic_missed_stream_messages_thread_friendly(False)
+
     @override_settings(EMAIL_GATEWAY_PATTERN="")
     def test_reply_warning_in_missed_personal_messages(self) -> None:
         self._reply_warning_in_missed_personal_messages(False)