fix early loading issue

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Added `Content.blank` https://github.com/Textualize/textual/pull/6264
 
+### Fixed
+
+- Fixed exception when setting `loading` attribute before mount https://github.com/Textualize/textual/pull/6268
+
 ## [6.7.1] - 2025-12-1
 
 ### Fixed

src/textual/widget.py

@@ -1035,7 +1035,10 @@ def set_loading(self, loading: bool) -> None:
 
     def _watch_loading(self, loading: bool) -> None:
         """Called when the 'loading' reactive is changed."""
-        self.set_loading(loading)
+        if not self.is_mounted:
+            self.call_later(self.set_loading, loading)
+        else:
+            self.set_loading(loading)
 
     ExpectType = TypeVar("ExpectType", bound="Widget")
 

tests/test_loading.py

@@ -1,6 +1,6 @@
 from textual.app import App, ComposeResult
 from textual.containers import VerticalScroll
-from textual.widgets import Label
+from textual.widgets import Label, Static
 
 
 class LoadingApp(App[None]):
@@ -31,3 +31,25 @@ async def test_loading_disables_and_remove_scrollbars():
         await pilot.pause()
 
         assert vs._check_disabled()
+
+
+async def test_premature_loading():
+    """Test a widget can set the `loading` attribute before mounting."""
+
+    # No assert, we're just expecting it to not crash
+
+    class LoadingWidget(Static):
+        """A widget that shows a loading indicator."""
+
+    class LoadingApp(App[None]):
+        """Simple app with a single loading widget."""
+
+        def compose(self) -> ComposeResult:
+            """Compose the app with the loading widget."""
+            widget = LoadingWidget("Loading content...")
+            widget.loading = True  # Should not crash
+            yield widget
+
+    app = LoadingApp()
+    async with app.run_test() as pilot:
+        await pilot.pause()