bugfix: model: Improve handling of _have_last_message.

This dict tracks whether the app has the last message in a narrow; this
commit fixes the following issues:
* There are too many narrows to pre-populate the dict, so ensure we use
  `.get` when checking it upon receiving a message event.
* The value for a narrow was set on each get_message call, but the value
  from a call is whether the returned messages contain the last message,
  so use both the previous state (if any) as well as the new state.

Author: neiljp

zulipterminal/model.py

@@ -461,13 +461,18 @@ def get_messages(self, *,
             if first_anchor and response['anchor'] != 10000000000000000:
                 self.index['pointer'][narrow_str] = response['anchor']
             if 'found_newest' in response:
-                self._have_last_message[narrow_str] = response['found_newest']
+                just_found_last_msg = response['found_newest']
             else:
                 # Older versions of the server does not contain the
                 # 'found_newest' flag. Instead, we use this logic:
                 query_range = num_after + num_before + 1
-                self._have_last_message[narrow_str] = (
-                    len(response['messages']) < query_range)
+                just_found_last_msg = len(response['messages']) < query_range
+
+            had_last_msg = self._have_last_message.get(narrow_str, False)
+            self._have_last_message[narrow_str] = (
+                had_last_msg or just_found_last_msg
+            )
+
             return ""
         display_error_if_present(response, self.controller)
         return response['msg']
@@ -980,7 +985,7 @@ def _handle_message_event(self, event: Event) -> None:
             set_count([message['id']], self.controller, 1)
 
         if (hasattr(self.controller, 'view')
-                and self._have_last_message[repr(self.narrow)]):
+                and self._have_last_message.get(repr(self.narrow), False)):
             msg_log = self.controller.view.message_view.log
             if msg_log:
                 last_message = msg_log[-1].original_widget.message