Setting self.dark in on_load doesn't work: No Screens on stack

[willmcgugan]

Suspect we could make this work.

[davep]

Minimal code to demonstrate the issue:

from textual.app        import App, ComposeResult
from textual.containers import Container
from textual.widgets    import Header, Footer

class EarlyDark( App[ None ] ):

    CSS = """
    Container {
        background: $panel;
        border: solid $panel-lighten-2;
    }
    """

    BINDINGS = [
        ( "d", "toggle", "Toggle Light/Dark" )
    ]

    def compose( self ) -> ComposeResult:
        yield Header()
        yield Container()
        yield Footer()

    def action_toggle( self ) -> None:
        self.dark = not self.dark

    def on_load( self ) -> None:
        self.dark = False

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

The result of running this is the following error on startup:

╭──────────────────────────────────────────────────────────── Traceback (most recent call last) ─────────────────────────────────────────────────────────────╮
│ /Users/davep/develop/python/textual-sandbox/early_dark.py:27 in on_load                                                                                    │
│                                                                                                                                                            │
│   24 │   │   self.dark = not self.dark                                                                                                                     │
│   25 │                                                                                                                                                     │
│   26 │   def on_load( self ) -> None:                                                                                                                      │
│ ❱ 27 │   │   self.dark = False                                                                                                                             │
│   28                                                                                                                                                       │
│   29 if __name__ == "__main__":                                                                                                                            │
│   30 │   EarlyDark().run()                                                                                                                                 │
│                                                                                                                                                            │
│ ╭─────────────────────────── locals ───────────────────────────╮                                                                                           │
│ │ self = EarlyDark(title='EarlyDark', classes={'-light-mode'}) │                                                                                           │
│ ╰──────────────────────────────────────────────────────────────╯                                                                                           │
│                                                                                                                                                            │
│ /Users/davep/develop/python/textual-sandbox/.venv/lib/python3.10/site-packages/textual/app.py:463 in watch_dark                                            │
│                                                                                                                                                            │
│    460 │   │   """Watches the dark bool."""                                                                                                                │
│    461 │   │   self.set_class(dark, "-dark-mode")                                                                                                          │
│    462 │   │   self.set_class(not dark, "-light-mode")                                                                                                     │
│ ❱  463 │   │   self.refresh_css()                                                                                                                          │
│    464 │                                                                                                                                                   │
│    465 │   def get_driver_class(self) -> Type[Driver]:                                                                                                     │
│    466 │   │   """Get a driver class for this platform.                                                                                                    │
│                                                                                                                                                            │
│ ╭─────────────────────────── locals ───────────────────────────╮                                                                                           │
│ │ dark = False                                                 │                                                                                           │
│ │ self = EarlyDark(title='EarlyDark', classes={'-light-mode'}) │                                                                                           │
│ ╰──────────────────────────────────────────────────────────────╯                                                                                           │
│                                                                                                                                                            │
│ /Users/davep/develop/python/textual-sandbox/.venv/lib/python3.10/site-packages/textual/app.py:1627 in refresh_css                                          │
│                                                                                                                                                            │
│   1624 │   │   stylesheet.set_variables(self.get_css_variables())                                                                                          │
│   1625 │   │   stylesheet.reparse()                                                                                                                        │
│   1626 │   │   stylesheet.update(self.app, animate=animate)                                                                                                │
│ ❱ 1627 │   │   self.screen._refresh_layout(self.size, full=True)                                                                                           │
│   1628 │                                                                                                                                                   │
│   1629 │   def _display(self, screen: Screen, renderable: RenderableType | None) -> None:                                                                  │
│   1630 │   │   """Display a renderable within a sync.                                                                                                      │
│                                                                                                                                                            │
│ ╭────────────────────────────────────────────────── locals ──────────────────────────────────────────────────╮                                             │
│ │    animate = True                                                                                          │                                             │
│ │       self = EarlyDark(title='EarlyDark', classes={'-light-mode'})                                         │                                             │
│ │ stylesheet = <Stylesheet                                                                                   │                                             │
│ │              │   [                                                                                         │                                             │
│ │              │   │   '/Users/davep/develop/python/textual-sandbox/early_dark.py:EarlyDark',                │                                             │
│ │              │   │   '/Users/davep/develop/python/textual-sandbox/.venv/lib/python3.10/site-packages/t'+17 │                                             │
│ │              │   ]                                                                                         │                                             │
│ │              >                                                                                             │                                             │
│ ╰────────────────────────────────────────────────────────────────────────────────────────────────────────────╯                                             │
│                                                                                                                                                            │
│ /Users/davep/develop/python/textual-sandbox/.venv/lib/python3.10/site-packages/textual/app.py:513 in screen                                                │
│                                                                                                                                                            │
│    510 │   │   try:                                                                                                                                        │
│    511 │   │   │   return self._screen_stack[-1]                                                                                                           │
│    512 │   │   except IndexError:                                                                                                                          │
│ ❱  513 │   │   │   raise ScreenStackError("No screens on stack") from None                                                                                 │
│    514 │                                                                                                                                                   │
│    515 │   @property                                                                                                                                       │
│    516 │   def size(self) -> Size:                                                                                                                         │
│                                                                                                                                                            │
│ ╭─────────────────────────── locals ───────────────────────────╮                                                                                           │
│ │ self = EarlyDark(title='EarlyDark', classes={'-light-mode'}) │                                                                                           │
│ ╰──────────────────────────────────────────────────────────────╯                                                                                           │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
ScreenStackError: No screens on stack

[davep]

Fixed by #1421.

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