This is a feature that you have to go out of your way to enable, not the default...

fenomas · on Dec 25, 2019

There's a setting for having multiple focus targets? Where?

I know there's a setting to prevent the keyboard from affecting dialogs at all, but obviously that's not what my comment was about.

ratww · on Dec 25, 2019

Using tab/space to navigate dialog buttons (like you're describing in your comment) is not a default setting in macOS. You have to explicitly enable it.

This is why it's complicated: it's not aimed at regular users, only at people who need it for accessibility reasons, or power users.

fenomas · on Dec 26, 2019

It's not complicated, it's just implemented in such a way that it leads to bad UX!

(One wonders: if the tab system didn't make two focuses, whether it wouldn't be intuitive enough to be on by default :D)