I have noticed quite a few times that after viewing a particular image and quitting Preview manually, Preview still remains active in the background sucking up huge chunks memory/RAM. It only stops after force quitting it. I don't know why this does happen!
This happens a lot. Preview is supposed to auto-terminate – but often cannot. As of this writing: this is a bug not fixed in Sierra and not fixed in High Sierra. Even force quitting the app from the Dock might surprise you: after one trigger-the-bug files hangs Preview, force quit the app from the Dock and open Activity Monitor. Even though the app has lost its indicator light in the Dock and will not show up if you switch through your applications with Cmd+Tab it may very well be still shown as running– and sometimes with quite a hefty CPU toll. Since this is a bug in all versions of Preview that are supposed to be able to be auto-terminated, you have not many options:
- disable auto-termination to mitigate some of these effects and get a bit more manual control over the situation with
defaults write -g NSDisableAutomaticTermination -bool yes
- if it happens: force-quit the app(-process) with Activity Monitor
- or do it from Terminal:
sudo killall Preview
Best option of course is not to use the buggy program and choose an alternative picture and PDF viewer.
This is by design and expected behavior if your memory pressure is green in activity monitor. Both iOS and macOS will keep some apps resident until compressed memory or paging is called for and then purge them. Changes in the kernel relating to App Nap, sudden termination, background processing allow many more states for applications than just running and quit.
Why do you care whether the process is still in memory? It's not running. Preview.app supports Automatic Termination, the system will remove it from memory when it is necessary to to so.