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


class ViewportUnits(App):
    CSS = """Static {width: 100vw; height: 100vh; border: solid cyan;} """

    def compose(self) -> ComposeResult:
        yield Static("Hello, world!")


app = ViewportUnits()
if __name__ == '__main__':
    app.run()