Computing is a "man-machine" system. Everything about a human interface has a human component; human physiology is what drives the requirements for the hardware and software. You can't talk about the hardware or user interface and exclude half of the equation.
The site has tags to cover topics like ergonomics and work environment. For those, it's easy to see that you can't talk about the best way to set up the computer if you leave out the user. Ergonomic keyboards are also on-topic. People who pound a keyboard all day don't want to get carpal tunnel syndrome, so that makes it on-topic.
It doesn't take long to get to a grey area. The QWERTY and Dvorak keyboard layouts, mice, GUI interfaces, etc. were all based on human requirements. The same with response time requirements for interface devices, visual user interfaces and web pages. Two recent questions come to mind.
How fast does application response time need to be The gist of this question and answer focused on people expending a lot of effort and money on the hardware and software side of the user interface to improve response time while being unaware of the role that the user plays in the result. This was downvoted even after eliminating the possibility of opinion-based answers because it focused on the user rather than the hardware/software, which may seem off-topic.
Why do i sometimes type the as teh mean that the response of keyboard is too was just closed as off-topic. The gist of one comment (since deleted), was a common sentiment--"Are you kidding me? You're asking on SU why you make typos? Learn to type." If you aren't familiar with human factors research, that reaction is understandable. It happens that the kind of error the question was about is caused by a poor user interface, and limitations of human physiology bumping up against that. In a sense, it isn't a whole lot different from users developing carpal tunnel syndrome from using a mismatched input device.
The cause is the interaction between the human machine and the computer machine. The solution can come from either or both sides of the equation. You can modify the keyboard (which is exactly where the QWERTY layout came from when the shoe was on the other foot), and this would clearly be on-topic here. You can mitigate the problem through software, which is clearly on-topic here. Or you can try to do things on the human side to better adapt to the characteristics of the technology, which appears to be off-topic.
So big picture, at the very least, understanding the human side of the equation can facilitate a solution on the hardware/software side. However, when someone has a problem that would be on-topic because it involves hardware or software and there are hardware/software solutions, does it become off-topic to discuss solutions that focus on the user? Where do we draw the line?
What is on-topic and off-topic is driven by the community. This question is not a request to declare all human factors questions on-topic, so please don't downvote it on that basis. Rather, I want to raise the question, generate discussion, and let the community decide what the standard should be. The purpose of the question is to develop a guideline, whatever that turns out to be, including a potential decision to ban human factors questions and/or responses. So up or down votes on the question should reflect whether you think there should be discussion and official guidance rather than whether you are for or against human factors content on SU. Hopefully, there will be answers for and against and that is where subject matter preference should be voted.