don't update styles when not attached

Author: willmcgugan

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## Unreleased
+
+### Changed
+
+- Optimized startup
+
 ## [3.4.0] - 2025-06-14
 
 ### Fixed

src/textual/dom.py

@@ -1707,6 +1707,8 @@ def _update_styles(self) -> None:
 
         Should be called whenever CSS classes / pseudo classes change.
         """
+        if not self.is_attached:
+            return
         try:
             self.app.update_styles(self)
         except NoActiveAppError:
@@ -1727,7 +1729,7 @@ def add_class(self, *class_names: str, update: bool = True) -> Self:
         self._classes.update(class_names)
         if old_classes == self._classes:
             return self
-        if update and self.is_attached:
+        if update:
             self._update_styles()
         return self
 
@@ -1746,7 +1748,7 @@ def remove_class(self, *class_names: str, update: bool = True) -> Self:
         self._classes.difference_update(class_names)
         if old_classes == self._classes:
             return self
-        if update and self.is_attached:
+        if update:
             self._update_styles()
         return self