I think that the issue @billw
is describing is that where the mirrors pivot "in" (was at one time referred to as a "breakaway mirror") they don't lock themselves into a strong detent position. When the mirror is where it's supposed to live there is about 3mm-5mm of rotational play along the pivotal axis, as though the hole that the detent lives in is much bigger than whatever is meant to hold it in place. As a result of this the mirror doesn't vibrate up-and-down when driving, but if lightly bumped or a door is closed a bit too hard the mirror housing itself will move to a spot just out of its original position and make it necessary to re-adjust the in/out (← →) of the glass.
I mentioned this when the car was in for a warranty repair last week and was told that this is not unusual (they refused to use the word "normal", and that felt pretty intentional) and that the whole assembly was completely functional and wouldn't be replaced as a warranty or defect item. In response to this I walked through their lot and checked about a dozen Civics of all body types (coupe, sedan, hatch) and found that all but two of them exhibited the same issue as mine with varying degrees of severity. It seems that their supplier for mirror housings has inconsistent quality control, but it also shows that the Honda acceptance criteria for these parts isn't terribly strict.
Couple the loose mirrors with the seemingly endless vibration noises I seem to be chasing all over the interior of this car and it feels like a repeat of the 2013 Civic and its low initial quality complaints.
I'm coping because - aside from the little issues - I really like the car a lot.