Oh, I think it's expected that we get some amount of personal stuff done during the day, and don't feel the need to hide this from my coworkers or even boss. At the lower end, there are few "white collar" desk jobs where you can't, say, take a phone call from a family member during the work day, or take coffee breaks whenever you want, or have to hide any of this from your boss.
If it got to the point where I felt like I had to hide the amount of time I was spending "working"/"not working" from my boss, then even if it were ethical (I still think it depends on the situation), I would just find it too stressful and unpleasant an experience, to be routinely hiding or lying about what I did with my day.
To me, doing 'some amount' of personal stuff during the day (which should be acceptable at any job), is still an entirely different thing than "I don't work Fridays, don't expect to see me online". Qualitatively, not just quantitatively.
"meeting output expectations" is tricky, I think, because what is the amount of "output" expected from "a person week"? It obviously varies for different people, at different times in their career, different skills, and just from week to week different things going on their lives/stress level. Knowing that I am not "cheating" my week is part of what makes me feel justified and honest in pushing back if a workplace does try to assign more work than is possible. Barring emergencies (and "your failure to plan is not my emergency") -- we should not be expected to work extra hours to get more work done. We get done in a week what we get done in a week, and then we come back the next week and do more -- to some extent, as engineers, the amount we can get done in a week is simply defined by what we do get done in a week.
But still, my real point is that even using this humane "you get done what you get done" approach, in our line of work, most organizations would find, I predict, that people working only 4 days a week (80% time), get done 90% or more of what people working five days a week do.
