Revisit and review the makeup of TabbedContent and its dependant parts

[davep]

After the recent review/revamp of containers, it's looking like ContentSwitcher and related things, such as TabbedContent, might have suffered a wee bit in terms of unintended consequences. See #2408 as an example. To borrow the code from there, this:

from textual.app import App, ComposeResult
from textual.widgets import TabbedContent, TabPane, Label
from textual.containers import Vertical, Horizontal

class ExampleApp(App):

    def compose(self) -> ComposeResult:
        with TabbedContent():

            with TabPane('a'):
                yield Label('1')
                yield Label('2')
                yield Label('3')

            with TabPane('b'):
                with Vertical():
                    yield Label('1')
                    yield Label('2')
                    yield Label('3')

            with TabPane('c'):
                with Vertical():
                    with Horizontal():
                        yield Label('1')
                    with Horizontal():
                        yield Label('2')
                    with Horizontal():
                        yield Label('3')

            with TabPane('d'):
                yield Label('1')
                with Vertical():
                    yield Label('2')
                    yield Label('3')
                yield Label('4')

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

behaves in unexpected ways out of the box. Styling with:

TabbedContent ContentSwitcher {
    height: 1fr;
}

has the code working in a more expected way.

I think we need to go back over the container-oriented parts of this chain of widgets to be totally sure we've now got the best choices of parent classes and widget-specific default styling.

Credit to @ankraft for identifying the problem, creating a nice self-contained example and reporting it.

[davep]

We should probably move this into TODO as soon as possible; this one seems to be biting a lot of people. Seems to be turning into quite an FAQ.

[github-actions]

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