fix for reactive vars not ready

src/textual/app.py

@@ -642,6 +642,7 @@ class App(Generic[ReturnType], DOMNode):
             )
 
         async def run_app() -> None:
+
             if quit_after is not None:
                 self.set_timer(quit_after, self.shutdown)
             if press is not None:
@@ -1100,6 +1101,7 @@ class App(Generic[ReturnType], DOMNode):
             mount_event = events.Mount(sender=self)
             await self._dispatch_message(mount_event)
 
+            Reactive.initialize_object(self)
             self.title = self._title
             self.stylesheet.update(self)
             self.refresh()

src/textual/message_pump.py

@@ -286,6 +286,7 @@ class MessagePump(metaclass=MessagePumpMeta):
 
     def _start_messages(self) -> None:
         """Start messages task."""
+        Reactive.initialize_object(self)
         self._task = asyncio.create_task(self._process_messages())
 
     async def _process_messages(self) -> None:
@@ -303,7 +304,6 @@ class MessagePump(metaclass=MessagePumpMeta):
         """Process messages until the queue is closed."""
         _rich_traceback_guard = True
 
-        await Reactive.initialize_object(self)
         while not self._closed:
             try:
                 message = await self._get_message()

src/textual/reactive.py

@@ -83,7 +83,7 @@ class Reactive(Generic[ReactiveType]):
         return cls(default, layout=False, repaint=False, init=True)
 
     @classmethod
-    async def initialize_object(cls, obj: object) -> None:
+    def initialize_object(cls, obj: object) -> None:
         """Call any watchers / computes for the first time.
 
         Args: