diff --git a/CHANGELOG.md b/CHANGELOG.md
index ce01e8bf9a..3518374a5e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Fixed `Tree` and `DirectoryTree` horizontal scrolling off-by-2 https://github.com/Textualize/textual/pull/4744
 - Fixed text-opacity in component styles https://github.com/Textualize/textual/pull/4747
 - Ensure `Tree.select_node` sends `NodeSelected` message https://github.com/Textualize/textual/pull/4753
+- Fixed message handlers not working when message types are assigned as the value of class vars https://github.com/Textualize/textual/pull/3940
 - Fixed `CommandPalette` not focusing the input when opened when `App.AUTO_FOCUS` doesn't match the input https://github.com/Textualize/textual/pull/4763
 - `SelectionList.SelectionToggled` will now be sent for each option when a bulk toggle is performed (e.g. `toggle_all`). Previously no messages were sent at all. https://github.com/Textualize/textual/pull/4759
 
diff --git a/src/textual/message.py b/src/textual/message.py
index 5eee60d8fd..383eb34d33 100644
--- a/src/textual/message.py
+++ b/src/textual/message.py
@@ -74,8 +74,16 @@ def __init_subclass__(
             cls.no_dispatch = no_dispatch
         if namespace is not None:
             cls.namespace = namespace
-        name = camel_to_snake(cls.__name__)
-        cls.handler_name = f"on_{namespace}_{name}" if namespace else f"on_{name}"
+            name = f"{namespace}_{camel_to_snake(cls.__name__)}"
+        else:
+            # a class defined inside of a function will have a qualified name like func.<locals>.Class,
+            # so make sure we only use the actual class name(s)
+            qualname = cls.__qualname__.rsplit("<locals>.", 1)[-1]
+            # only keep the last two parts of the qualified name of deeply nested classes
+            # for backwards compatibility, e.g. A.B.C.D becomes C.D
+            namespace = qualname.rsplit(".", 2)[-2:]
+            name = "_".join(camel_to_snake(part) for part in namespace)
+        cls.handler_name = f"on_{name}"
 
     @property
     def control(self) -> DOMNode | None:
diff --git a/src/textual/message_pump.py b/src/textual/message_pump.py
index bce43092c9..108c51b4cf 100644
--- a/src/textual/message_pump.py
+++ b/src/textual/message_pump.py
@@ -11,7 +11,6 @@
 from __future__ import annotations
 
 import asyncio
-import inspect
 import threading
 from asyncio import CancelledError, Queue, QueueEmpty, Task, create_task
 from contextlib import contextmanager
@@ -36,7 +35,6 @@
 from ._context import prevent_message_types_stack
 from ._on import OnNoWidget
 from ._time import time
-from .case import camel_to_snake
 from .css.match import match
 from .errors import DuplicateKeyHandlers
 from .events import Event
@@ -78,8 +76,6 @@ def __new__(
         class_dict: dict[str, Any],
         **kwargs: Any,
     ) -> _MessagePumpMetaSub:
-        namespace = camel_to_snake(name)
-        isclass = inspect.isclass
         handlers: dict[
             type[Message], list[tuple[Callable, dict[str, tuple[SelectorSet, ...]]]]
         ] = class_dict.get("_decorated_handlers", {})
@@ -93,13 +89,6 @@ def __new__(
                 ] = getattr(value, "_textual_on")
                 for message_type, selectors in textual_on:
                     handlers.setdefault(message_type, []).append((value, selectors))
-            if isclass(value) and issubclass(value, Message):
-                if "namespace" in value.__dict__:
-                    value.handler_name = f"on_{value.__dict__['namespace']}_{camel_to_snake(value.__name__)}"
-                else:
-                    value.handler_name = (
-                        f"on_{namespace}_{camel_to_snake(value.__name__)}"
-                    )
 
         # Look for reactives with public AND private compute methods.
         prefix = "compute_"
diff --git a/tests/test_message_handling.py b/tests/test_message_handling.py
index 31edeb2cf4..3215ab6deb 100644
--- a/tests/test_message_handling.py
+++ b/tests/test_message_handling.py
@@ -24,6 +24,11 @@ class Right(BaseWidget):
         class Fired(BaseWidget.Fired):
             pass
 
+    class DummyWidget(Widget):
+        # ensure that referencing a message type in other class scopes
+        # doesn't break the namespace
+        _event = Left.Fired
+
     handlers_called = []
 
     class MessageInheritanceApp(App[None]):