fix(tabbed content): handle pane containing tabs

[TomJGooding]

Fixes #3412. There might be a better way of discerning Tabs messages, so creating only as a draft for now.

I would still need to add regression tests, but this seems to work from some quick manual testing with the app below:

from textual.app import App, ComposeResult
from textual.widgets import TabbedContent, TabPane, Tabs


class TabsApp(App):
    BINDINGS = [("space", "clear_inner_tabs")]
    CSS = """
    TabPane > Tabs {
        height: auto;
    }
    """

    def compose(self) -> ComposeResult:
        with TabbedContent():
            with TabPane("outer tab #1"):
                yield Tabs(
                    "inner tab#1",
                    "inner tab#2",
                    "inner tab#3",
                    id="inner-tabs-one",
                )
            with TabPane("outer tab #2"):
                yield Tabs(
                    "inner tab#4",
                    "inner tab#5",
                    "inner tab#6",
                    id="inner-tabs-two",
                )

    def action_clear_inner_tabs(self) -> None:
        self.query_one("#inner-tabs-one", Tabs).clear()
        self.query_one("#inner-tabs-two", Tabs).clear()


if __name__ == "__main__":
    app = TabsApp()
    app.run()

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[TomJGooding]

There's obviously other events such as tabs_cleared that might be trickier to handle.

Sorry clearly I failed to heed my own advice before creating this PR... I'll have a a closer look at the code to understand the failing tests.

[TomJGooding]

I think this docstring is misleading, as Tabs.Cleared is actually sent when when there are no active tabs.

textual/src/textual/widgets/_tabbed_content.py

Lines 393 to 394 in e4d182d

	def _on_tabs_cleared(self, event: Tabs.Cleared) -> None:
	"""All tabs were removed."""

[TomJGooding]

I must be still missing something obvious, as the same tests fail after changing to:

--- a/src/textual/widgets/_tabbed_content.py
+++ b/src/textual/widgets/_tabbed_content.py
@@ -395,10 +395,10 @@ class TabbedContent(Widget):
         )
 
     def _on_tabs_cleared(self, event: Tabs.Cleared) -> None:
-        """All tabs were removed."""
-        if self.get_child_by_type(Tabs).tab_count != 0:
+        """There are no active tabs."""
+        if self.get_child_by_type(Tabs).active_tab is not None:
             # A tab pane could contain its own Tabs widget, so just return
-            # early if the TabbedContent still contains tabs.
+            # early if the TabbedContent still contains active tabs.
             # https://github.com/Textualize/textual/issues/3412
             return
         event.stop()

[TomJGooding]

I hoped this might be fixed with the changes to Tabs/TabbedContent done in #3498, but unfortunately I'm still seeing the same test failures.

I'm not sure this is really the best way to discern Tabs messages within TabbedContent anyway. Sorry @darrenburns to tag you, but before I throw in the towel and close this PR, do you have any ideas on this?

[TomJGooding]

Closed in favour of #3602.