Earlier quoted context omitted.
Focus stealing has been an issue in windowed multi-tasking environments from the beginning. It's certainly been an issue in all macOS/OS X versions I've used since I started in 2011.
This bug isn't that. It's the frontmost app losing focus to nothing.
Just install the SuperTyping app. It's sooo good and intuitive. Totally worth the $189, if you consider how often you need to type something.
I also recommend Little Snitch as firewall and Parallels for virtualization.
Does anyone have a recommendation for bootloader or filesystem app? Preferably subscription model for intuitive accounting.