Children can be added to ALLOW_CHILDREN=False widgets

[davep]

Raising this after going over it during the close of 2023-11-29 standup

Following on from #3758, which ALLOW_CHILDREN was introduced, and where code like this:

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

class NoChildren1(App[None]):

    def compose(self) -> ComposeResult:
        with Static():
            yield Static("Hello")

if __name__ == "__main__":
    NoChildren1().run()

will not result in a NotAContainer exception... it is still possible to treat such a widget as a container, either by inheriting from it and composing in the widgets:

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

class DodgyStatic(Static):

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

class NoChildren2(App[None]):

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

if __name__ == "__main__":
    NoChildren2().run()

by mounting widgets into it:

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

class NoChildren3(App[None]):

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

    def on_mount(self) -> None:
        self.query_one(Static).mount(Static("Hello"))

if __name__ == "__main__":
    NoChildren3().run()

or by passing children to a widget that accepts children but has ALLOW_CHILDREN=False:

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

class NotACounter(Widget):
    ALLOW_CHILDREN = False

class NoChildren4(App[None]):

    def compose(self) -> ComposeResult:
        yield NotACounter(Static("Hello"))

if __name__ == "__main__":
    NoChildren4().run()

If ALLOW_CHILDREN were to affect all these cases it would also mean that feature would pretty much satisfy (albeit there would be a need for an internal override) a long-standing issue: #2210 (which in turn goes back a bit further).

[willmcgugan]

Redundant now, since we reversed the ALLOW_CHILDREN idea.

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