If you assign a message type to a class variable belonging to a class that derives from MessagePump, non-decorated message handlers stop working because the message type's handler_name incorrectly gets reassigned to use the name of the class containing the class variable as the namespace.

I ran into this when writing a general class for input prompts/modals, where I was using class variables to specify what event to treat as the "submit" event.

Simple example, where on_button_pressed won't be run unless EVENT_TYPE is removed:

from textual.app import App, ComposeResult
from textual.widgets import Button


class MyApp(App):
    # prevents on_button_pressed from running
    EVENT_TYPE = Button.Pressed

    def compose(self) -> ComposeResult:
        yield Button("Click me")

    def on_button_pressed(self) -> None:
        self.query_one(Button).label = "Clicked!"


if __name__ == '__main__':
    MyApp().run()
    # prints "on_my_app_pressed"!
    print(Button.Pressed.handler_name)