boxes: Add class method in MessageBox to update content header.

This commit adds a method to update content header of a specific message
box (if required).

The current logic re-builds the whole message box if an update is required.
This might need improvement as to update a specific element only. (Though
that might prove useful only if that element is rendered via a dedicated
class method).

Tests added.

Author: Ezio-Sarthak

tests/ui/test_ui_tools.py

@@ -2285,6 +2285,49 @@ def test_main_view_generates_EDITED_label(
             assert label[0].text == "EDITED"
             assert label[1][1] == 7
 
+    @pytest.mark.parametrize(
+        "message",
+        [
+            {
+                "id": 4,
+                "type": "stream",
+                "display_recipient": "Verona",
+                "stream_id": 5,
+                "subject": "Test topic",
+                "is_me_message": True,
+                "flags": [],
+                "content": "",
+                "reactions": [],
+                "sender_full_name": "Human 1",
+                "timestamp": 1532103879,
+            }
+        ],
+    )
+    @pytest.mark.parametrize(
+        "to_vary_in_each_message, header_needed, update_required",
+        [
+            ({"subject": "Test topic 2"}, True, True),
+            ({}, False, False),
+            ({"sender_full_name": "Human 2"}, False, True),
+            ({"timestamp": 1532103869}, False, False),
+        ],
+        ids=[
+            "recipient_header_present",
+            "recipient_header_not_present",
+            "content_header_present",
+            "content_header_not_present",
+        ],
+    )
+    def test_update_msg_content_header(
+        self, msg_box, message, update_required, mocker, to_vary_in_each_message,
+        header_needed
+    ):
+        last_msg = dict(message, **to_vary_in_each_message)
+
+        msg_box = MessageBox(message, self.model, last_msg)
+
+        assert msg_box.update_msg_content_header() == update_required
+
     @pytest.mark.parametrize("key", keys_for_command("STREAM_MESSAGE"))
     @pytest.mark.parametrize(
         "narrow, expect_to_prefill",

zulipterminal/ui_tools/boxes.py

@@ -1361,6 +1361,26 @@ def main_view(self) -> List[Any]:
         ]
         return [part for part, condition in parts if condition]
 
+    def update_msg_content_header(self) -> bool:
+        """
+        Update the content header of a message box (if required).
+        """
+        update_required = False
+
+        if self.need_recipient_header():
+            update_required = self[1][1].text != " "
+
+        # Content header is not None
+        elif isinstance(self[0][1], urwid.Text):
+            update_required = self[0][1].text != " "
+
+        if update_required:
+            # Re initialize the message if update is required.
+            # FIXME: Render specific element (here content header) instead?
+            super().__init__(self.main_view())
+
+        return update_required
+
     @classmethod
     def transform_content(
         cls, content: Any, server_url: str