buttons/helper: Add support for topic narrow in handle_narrow.

zulipterminal/helper.py

@@ -11,6 +11,7 @@
     Any, Callable, DefaultDict, Dict, FrozenSet, Iterable, List, Set, Tuple,
     TypeVar, Union,
 )
+from urllib.parse import unquote
 
 import lxml.html
 from mypy_extensions import TypedDict
@@ -618,3 +619,9 @@ def display_error_if_present(response: Dict[str, Any], controller: Any
                              ) -> None:
     if response['result'] == 'error':
         controller.view.set_footer_text(response['msg'], 3)
+
+
+def hash_util_decode(string: str) -> str:
+    # Acknowledge custom string replacements in zulip/zulip's
+    # zerver/lib/url_encoding.py before unquote.
+    return unquote(string.replace('.', '%').replace('%2E', '.'))

zulipterminal/ui_tools/buttons.py

@@ -3,6 +3,7 @@
 import urwid
 
 from zulipterminal.config.keys import is_command_key, keys_for_command
+from zulipterminal.helper import hash_util_decode
 from zulipterminal.urwid_types import urwid_Size
 
 
@@ -296,9 +297,8 @@ def handle_link(self, *_: Any) -> None:
 
     def handle_narrow(self) -> None:
         """
-        Handles stream narrow.
+        Handles stream and topic narrow.
         """
-        # TODO: Handle topic narrow.
         link_split = self.link.split('/')
 
         self.stream_id = int(link_split[5].split('-')[0])
@@ -311,7 +311,11 @@ def handle_narrow(self) -> None:
 
         self.stream_name = self.model.stream_dict[self.stream_id]['name']
 
-        if '/stream/' in self.link:
+        if '/topic/' in self.link:
+            encoded_topic = link_split[7]
+            self.topic_name = hash_util_decode(encoded_topic)
+            self.controller.narrow_to_topic(self)
+        elif '/stream/' in self.link:
             self.controller.narrow_to_stream(self)
 
         # Exit popup if the narrow was successful.