If you're like me and you have a 1440p monitor that you like to run in 1080p HiDPI mode using a custom plist or SwitchResX, DO NOT UPGRADE TO SIERRA. Likewise if you have a 4K display and you like a bit more space while maintaining that sweet retina look and so you run it in 1440p HiDPI, DO NOT UPGRADE TO SIERRA. Apple seems to have inexplicably removed this ability. You're now forced to destroy your eyesight by running the Dell U2515H at native resolution (Windows, believe it or not, handles 1440p native better than MacOSX does on this monitor). One final thing - if you are a 4K or 1440p screen user and are on MacOS Sierra 10.12.1 BETA that came out today, please could you test using this method to see if they have restored this functionality? Otherwise I'm going to have to downgrade to El Capitan forever.