Fix: make widget.loading thread-safe (closes #5108)

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
+
+### Fixed
+
+- Fixed `loading` reactive not being thread-safe when set from a worker thread https://github.com/Textualize/textual/issues/5108
+
 ## [8.2.8] - 2026-06-30
 
 ### Fixed

src/textual/widget.py

@@ -5,6 +5,8 @@
 
 from __future__ import annotations
 
+import threading
+
 from asyncio import create_task, gather, wait
 from collections import Counter
 from contextlib import asynccontextmanager
@@ -1074,7 +1076,12 @@ def _watch_loading(self, loading: bool) -> None:
         if not self.is_mounted:
             self.call_later(self.set_loading, loading)
         else:
-            self.set_loading(loading)
+            if threading.get_ident() == self.app._thread_id:
+                self.set_loading(loading)
+            else:
+                # Setting `loading` from a worker thread: hop over to the
+                # app's thread instead of touching the UI directly.
+                self.app.call_from_thread(self.set_loading, loading)
 
     ExpectType = TypeVar("ExpectType", bound="Widget")
 

tests/test_loading.py

@@ -53,3 +53,25 @@ def compose(self) -> ComposeResult:
     app = LoadingApp()
     async with app.run_test() as pilot:
         await pilot.pause()
+
+
+async def test_loading_set_from_worker_thread():
+    """Regression test for #5108: setting `loading` from a background
+    thread should not touch the DOM directly, and should not raise."""
+
+    class LoadingApp(App[None]):
+        def compose(self) -> ComposeResult:
+            yield Label("hello")
+
+    app = LoadingApp()
+    async with app.run_test() as pilot:
+        label = app.query_one(Label)
+
+        def set_loading_from_thread() -> None:
+            label.loading = True
+
+        app.run_worker(set_loading_from_thread, thread=True)
+        await pilot.pause()
+        await pilot.pause()
+
+        assert label.loading is True