Nathan,
If you are using an imported Work Environment that was created in a previous version try switching to the Standard Profile 16. Options / Work Environment / Work Environment / Work Environment Profiles. Does the quick selection option return to normal? If so then you should re-build your work environment for 16.
If not then do a New & Reset All. Go to File / New and hold the alt key and you will see that the New file command changes to read "New & Reset All". This will open a new blank file and clear out some preferences that may have gotten out of whack. Then re-open one of your projects and see if the trouble is gone.
If not then perform a full Preferences Cleaning to clear out a larger segment of cache files that may be corrupted somewhere:
http://archicadwiki.com/PreferencesCleaning
Please let me know if you have any questions.
Best regards,