Figured it out! It's google pages in Safari. For example inbox, or gmail, or google calendar. If one of those is loaded in Safari after about 20 seconds F4 will stop functioning. Verified this on a second MacBook Pro late 2016 with Touch Bar. Steps to reproduce: 1) Open Keyboard Preferences 2) Keyboard tab 3) Ensure "Show keyboard and emoji viewers in menu bar" is enabled 4) Go to menubar and unde