Make hovered tabs be drawn with the unselected's width at minimum

[YeldhamDev]

Fixes #81836. Supersedes #82344.

This PR also fixes a small bug I found that makes tabs still be hovered if mouse exits the window.

[YuriSizov]

Looks good to me. I wonder what other UI toolkits do in this case.

There is a small issue with the fact that we still use hover stylebox's margins, so the text alignment shifts. But I think this is fine for now so we fix the crash. Perhaps a bit of refactoring in TabBar in the future would make hover behave more like focus to avoid these issues.

[akien-mga]

Thanks!

[YeldhamDev]

@YuriSizov This is less an issue and more of "how it works". The text on tabs aren't drawn centered, they just take into account the content_margin of the styles, so if you have a hover style that doesn't have margins and an unselected one that does, this happens.

[YuriSizov]

@YeldhamDev I know, but if we are enforcing some styling from another stylebox, we should probably do it consistently and apply margins as well. This shouldn't be an issue for most, of course, since making them different size is pretty user-unfriendly in the first place. Which is why I'm only leaving it as a passing comment. But still, keeping some consistency would be nice.

[YeldhamDev]

Remember when I said that _draw_tab() is only aware of the style you give it and nothing more, and that changing it for such a corner case would not be worth the extra complexity? 🙃

[YuriSizov]

Cherry-picked for 4.1.2.