From c0910a67b5617122412483c98da9d5ae3c4719e1 Mon Sep 17 00:00:00 2001
From: Will McGugan <willmcgugan@gmail.com>
Date: Sun, 8 Jun 2025 15:07:00 +0100
Subject: [PATCH 1/2] sync primitives

---
 src/textual/app.py          | 17 ++++++++++++++++-
 src/textual/message_pump.py | 14 +++++++++++---
 src/textual/timer.py        |  1 +
 3 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/textual/app.py b/src/textual/app.py
index 478593124e..87370c5152 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -865,6 +865,17 @@ def __init_subclass__(cls, *args, **kwargs) -> None:
 
         return super().__init_subclass__(*args, **kwargs)
 
+    def _thread_init(self):
+        """Initialize threading primitives for the current thread.
+
+        https://github.com/Textualize/textual/issues/5845
+
+        """
+        self._message_queue
+        self._mounted_event
+        self._exception_event
+        self._thread_id = threading.get_ident()
+
     def _get_dom_base(self) -> DOMNode:
         """When querying from the app, we want to query the default screen."""
         return self.default_screen
@@ -2094,8 +2105,9 @@ async def run_auto_pilot(
                     run_auto_pilot(auto_pilot, pilot), name=repr(pilot)
                 )
 
+        self._thread_init()
+
         app._loop = asyncio.get_running_loop()
-        app._thread_id = threading.get_ident()
         with app._context():
             try:
                 await app._process_messages(
@@ -3120,6 +3132,9 @@ async def _process_messages(
         terminal_size: tuple[int, int] | None = None,
         message_hook: Callable[[Message], None] | None = None,
     ) -> None:
+
+        self._thread_init()
+
         async def app_prelude() -> bool:
             """Work required before running the app.
 
diff --git a/src/textual/message_pump.py b/src/textual/message_pump.py
index 7a1e3bde7d..19e37e4a19 100644
--- a/src/textual/message_pump.py
+++ b/src/textual/message_pump.py
@@ -11,7 +11,6 @@
 from __future__ import annotations
 
 import asyncio
-import sys
 import threading
 from asyncio import CancelledError, QueueEmpty, Task, create_task
 from contextlib import contextmanager
@@ -23,12 +22,10 @@
     Awaitable,
     Callable,
     Generator,
-    Generic,
     Iterable,
     Type,
     TypeVar,
     cast,
-    overload,
 )
 from weakref import WeakSet
 
@@ -163,6 +160,15 @@ def _prevent_message_types_stack(self) -> list[set[type[Message]]]:
             prevent_message_types_stack.set(stack)
         return stack
 
+    def _thread_init(self):
+        """Initialize threading primitives for the current thread.
+
+        Require for Python3.8 https://github.com/Textualize/textual/issues/5845
+
+        """
+        self._message_queue
+        self._mounted_event
+
     def _get_prevented_messages(self) -> set[type[Message]]:
         """A set of all the prevented message types."""
         return self._prevent_message_types_stack[-1]
@@ -506,6 +512,8 @@ async def _close_messages(self, wait: bool = True) -> None:
 
     def _start_messages(self) -> None:
         """Start messages task."""
+        self._thread_init()
+
         if self.app._running:
             self._task = create_task(
                 self._process_messages(), name=f"message pump {self}"
diff --git a/src/textual/timer.py b/src/textual/timer.py
index f4b7b1ac1c..af657b3b69 100644
--- a/src/textual/timer.py
+++ b/src/textual/timer.py
@@ -152,6 +152,7 @@ async def _run(self) -> None:
         count = 0
         _repeat = self._repeat
         _interval = self._interval
+        self._active  # Force instantiation in same thread
         await self._active.wait()
         start = _time.get_time()
 

From c6c8b8ea8bd489318e5164ce542d46a17250b4b2 Mon Sep 17 00:00:00 2001
From: Will McGugan <willmcgugan@gmail.com>
Date: Sun, 8 Jun 2025 15:13:02 +0100
Subject: [PATCH 2/2] test

---
 tests/test_message_pump.py | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/tests/test_message_pump.py b/tests/test_message_pump.py
index 58fb5d325a..e90b576a52 100644
--- a/tests/test_message_pump.py
+++ b/tests/test_message_pump.py
@@ -1,3 +1,5 @@
+import threading
+
 import pytest
 
 from textual._dispatch_key import dispatch_key
@@ -169,3 +171,22 @@ def on_button_pressed(self, event: Button.Pressed) -> None:
     async with app.run_test() as pilot:
         await pilot.click(MyButton)
         assert app_button_pressed
+
+
+async def test_thread_safe_post_message():
+    class TextMessage(Message):
+        pass
+
+    class TestApp(App):
+
+        def on_mount(self) -> None:
+            msg = TextMessage()
+            threading.Thread(target=self.post_message, args=(msg,)).start()
+
+        def on_text_message(self, message):
+            self.exit()
+
+    app = TestApp()
+
+    async with app.run_test() as pilot:
+        await pilot.pause()