This video shows how to fix a folding mirror that does not stop in the correct "open" position. Volvo's solution to this problem is to replace the entire mirror assembly, since normally this spring/pin is not serviceable. The cost is a whole new mirror housing, and at least 2h of labor. However, it is possible to fix yourself.

Tools required:
Small Torx bit (I think T10?)
Hammer (most likely)
Plastic Pry Set (recommended)
Pointed punch, or some pointed pry tools.
Wire brush/Dremel
Lubricant of some sort

Here is a good video showing how to remove those pesky press nuts:
   • Video  

Unfortunately I did not have the sturdy pick set.

Original "2012 Volvo XC60 Mirror Internals" Video here:
   • 2012 XC60 Mirror Internals