Duplicate CSS errors from screens #3581
Description
CSS errors in screens are still duplicate (see #3566, and https://github.com/Textualize/textual/pull/3566/files#r1367884524 in particular).
CSS parsing errors show up as duplicate when parsing the CSS class variable or css from an external file, but not from DEFAULT_CSS.
App to reproduce:
from textual.app import App
from textual.screen import Screen
class MyScreen(Screen[None]):
# CSS_PATH = "myscreen.tcss"
CSS = "Screen { screen-baz: spam; }"
class MyApp(App[None]):
def on_mount(self):
self.push_screen(MyScreen())
if __name__ == "__main__":
MyApp().run()