helper: Index case insensitive topic names by narrowed topic.

Currently, the topic indexing conditional in index_messages does case
sensitive comparison with narrowed topic and msg_topic which results in
similar topic names (case insensitive) not getting indexed at all.

This performs lowercase comparison and indexes messages by the narrowed
topic. For the latter part, if it finds a similar topic name already
indexed, it updates that topic name (key) with the matching [lowercase]
narrowed topic.

Consequently, topic narrow from the topic list (for the latest topic
name) or from 'All Messages'/stream narrow (for any of the similar topic
name), contains all the messages.

Author: preetmishra

zulipterminal/helper.py

@@ -406,11 +406,27 @@ def index_messages(messages: List[Message],
         if msg['type'] == 'stream' and len(narrow) == 2:
             narrow_topic = narrow[1][1]
             msg_topic = msg['subject']
-            if narrow_topic == msg_topic:
+            # Do a lowercase comparison to acknowledge case insensitive topic
+            # names. For instance, case, CASE and cAsE.
+            if narrow_topic.lower() == msg_topic.lower():
                 topics_in_stream = index['topic_msg_ids'][msg['stream_id']]
-                if not topics_in_stream.get(msg_topic):
-                    topics_in_stream[msg_topic] = set()
-                topics_in_stream[msg_topic].add(msg['id'])
+                lowercase_topic_names = map(str.lower, topics_in_stream)
+
+                # Update existing topic name key for the narrowed topic with
+                # narrow_topic.
+                # NOTE: lowercase_topic_names doesn't need to be updated.
+                for topic_name in topics_in_stream:
+                    if topic_name.lower() == narrow_topic.lower():
+                        existing_ids = topics_in_stream[topic_name]
+                        del topics_in_stream[topic_name]
+                        topics_in_stream[narrow_topic] = existing_ids
+                        break
+
+                # Use narrow_topic as a key in the index (topics_in_stream) to
+                # unify case insensitive topic names.
+                if msg_topic.lower() not in lowercase_topic_names:
+                    topics_in_stream[narrow_topic] = set()
+                topics_in_stream[narrow_topic].add(msg['id'])
 
     return index