Style changes made `on_mount` no longer applying correctly in 0.45

[davep]

Given this code:

from textual import __version__
from textual.app import App, ComposeResult
from textual.widgets import Static

class BrokenClassesApp(App[None]):

    CSS = """
    Screen {
        align: center middle;
    }

    Static {
        width: 50%;
        height: 50%;
        border: solid;
    }

    Screen.go-red Static {
        background: red;
    }
    """

    def compose(self) -> ComposeResult:
        yield Static(f"This should have a red background\n\nTextual {__version__}")

    def on_mount(self) -> None:
        self.screen.set_class(True, "go-red")

if __name__ == "__main__":
    BrokenClassesApp().run()

in Textual 0.44.x the result was as I'd expect:

but as of 0.45 it seems the class no longer applies:

Flagging @willmcgugan for triage as this seems like an unintended consequence of some of the CSS performance tweaks.

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