Handle other Tabs widgets as DOM descendants of a TabbedContent

[darrenburns]

The key change here is that when a TabbedContent creates its Tabs, it'll be able to easily identify messages from that instance.

This is important because TabbedContent should only be responding to messages from the Tabs it creates. Messages from other Tabs instances that may appear further down the DOM as descendants should be ignored.

Also fixes #3412, which was a crash.

[TomJGooding]

Thanks Darren, this looks like a better way of handling this. I still don't quite understand how Tabs would still have an active tab after a Cleared message though which is why tests were failing on my attempt #3444 (comment).

I think the docstring is a bit misleading for _on_tabs_cleared, as this message is posted when there are no active tabs, not when all tabs are removed, so perhaps that should be updated.

[darrenburns]

@TomJGooding I think on your attempt, the tab_count != 0 early return inside the tabs cleared wasn't correct, because a Cleared message is sent when all tabs are hidden (which feels wrong, but that's a different story).

So, when that watcher was called, the tab_count was actually 2, triggering the early return. Since this early return prevented TabbedContent.active from being updated, it remained stuck on its last value, and the test failed.

I don't think I've explained great, but the whole flow is very confusing with lots of messages, reactive updates, and side effects happening - it's really not easy to trace execution here.

[TomJGooding]

Sorry just to confirm I meant my change to the early return in #3444 (comment). I tried both active and active_tab (which I always mix up!) but still had the failing tests, suggesting it still has an active tab? I agree the flow is a pretty confusing, so maybe I'm just misunderstanding something here.

[darrenburns]

I think that's a race-condition. active_tab is updated after refresh (via call_after_refresh), so when you check it it may not have been updated yet. active is updated immediately, so maybe using active instead of active_tab would work there... but I'm not sure.

[darrenburns]

Actually - this doesn't work as the Tabs isn't a direct child of the TabbedContent. 😞

[TomJGooding]

Thanks Darren, this obviously was more involved than I expected from my attempt!

My only suggestion is could we also change this misleading docstring for _on_tabs_cleared please?