Fix issue with tabbed content underline

[rodrigogiraoserrao]

I did some investigation around #2229 and I figured out the issue is related to the fact that it takes a lot of time to swap in and out the contents of the tabbed content.
Commenting out this line makes the issue go away:

textual/src/textual/widgets/_tabbed_content.py

Line 188 in c249548

switcher.current = event.tab.id

That's called when the message Tabs.TabActivated is handled in TabbedContent, and that message is coming from Tabs.watch_active.

A simple fix would be to throttle Tabs.watch_active so that it doesn't get called too often.
That is what this PR does, and merging it will close #2229.

Another possible solution, that is more involved, is to add a task-based mechanism around swapping the visible sub-widget of the ContentSwitcher in ContentSwitch.watch_current, such that when we schedule a new switch, old ones get thrown away.

[willmcgugan]

This feels like it might be masking the error, rather than fixing it. I don't think you guarantee that time mechanism won't fail on a slower machine or with faster key repeat.

Have you established why this occurs?

[rodrigogiraoserrao]

This feels like it might be masking the error, rather than fixing it. [...]

Sadly, I share this sentiment.

Have you established why this occurs?

Not really... I will dig some more.

[rodrigogiraoserrao]

Closing this because the actual fix (coming soon) is unrelated to this throttling attempt.