Back in the old days, the concept of “focus,” that is, which active window, button, text field or icon being active or selected was very clear. If a message box or dialogue box appeared on the screen or “took focus” from whatever you were doing, it was clear that you’d have the option to either press space bar, tab, alt+key, or any other combination thereof to make your desired selection quickly and move on.
This was the norm for many years going back as far as MS-DOS to Windows98 SE if my memory serves. After that, somehow the rules changed. Mind you, I’m not talking about Linux here, just Windows. From the limited distros I’ve tested, Linux seems to be pretty spot-on with handling focus consistently.