Background queries on `App` when command palette is shown causes an exception

[davep]

Technically this isn't an issue with the command palette, it would be an issue with any screen with a background process that causes a query to be handled by the app, but via the command palette is the way that a dev would most likely encounter it and it's where they'd find it most surprising.

Consider this code:

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

class CommandPaletteIsolation(App[None]):

    def compose(self) -> ComposeResult:
        yield Log()

    def tick(self) -> None:
        self.query_one(Log).write_line("Tick")

    def on_mount(self) -> None:
        self.set_interval(1, self.tick)

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

Let it run for a moment, then summon the command palette and keep it up. The next time tick runs a NoMatches will result.

This version, where the default screen isn't being used, isn't a problem:

from textual.app import App, ComposeResult
from textual.screen import Screen
from textual.widgets import Log

class MyScreen(Screen):

    def compose(self) -> ComposeResult:
        yield Log()

    def tick(self) -> None:
        self.query_one(Log).write_line("Tick")

    def on_mount(self) -> None:
        self.set_interval(1, self.tick)

class CommandPaletteIsolation(App[None]):

    def on_mount(self) -> None:
        self.push_screen(MyScreen())

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

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