Add regression test for #1814.

tests/test_message_handling.py

@@ -0,0 +1,45 @@
+from textual.app import App, ComposeResult
+from textual.message import Message
+from textual.widget import Widget
+
+
+async def test_message_inheritance_namespace():
+    """Inherited messages get their correct namespaces.
+
+    Regression test for https://github.com/Textualize/textual/issues/1814.
+    """
+
+    class BaseWidget(Widget):
+        class Fired(Message):
+            pass
+
+        def trigger(self) -> None:
+            self.post_message(self.Fired())
+
+    class Left(BaseWidget):
+        class Fired(BaseWidget.Fired):
+            pass
+
+    class Right(BaseWidget):
+        class Fired(BaseWidget.Fired):
+            pass
+
+    handlers_called = []
+
+    class MessageInheritanceApp(App[None]):
+        def compose(self) -> ComposeResult:
+            yield Left()
+            yield Right()
+
+        def on_left_fired(self):
+            handlers_called.append("left")
+
+        def on_right_fired(self):
+            handlers_called.append("right")
+
+    app = MessageInheritanceApp()
+    async with app.run_test():
+        app.query_one(Left).trigger()
+        app.query_one(Right).trigger()
+
+    assert handlers_called == ["left", "right"]