I have a strange problem with my new MacBook Pro running Sierra. After visiting some web sites (such as stackexchange, gmail, and web.whatsapp.com), the F4 key stops to work in all applications. All other function keys continue working normally, as well as starting launchpad (which is also on the F4 key). It seems pressing the F4 key does not generate any key code anymore, according to Key Codes.

• This happens both with the internal MacBook keyboard as well as the apple wireless keyboard.
• Logging out and logging in again (or rebooting) fixes the F4 key, only quitting Safari does not fix it.
• Disabling Javascript (via Safari preferences) leaves the F4 key enabled.
• Other browsers (Chrome, Firefox) are not affected by this issue.
• It is a completely fresh installation of Sierra and happens without installing any additional programs or modifying any settings. Tested with 10.12.2 and 10.12.3.

Did anyone notice the same issue?

Does anyone have an idea how to debug or prevent this issue?

An easy way to test if you have this issue is by:

1. Log in.
2. Start the terminal.
3. Press Control-v and F4 in the terminal. It should print ^[OS.
4. Start Safari, go to stackexchange.com.
5. Repeat step 3, nothing gets printed on the terminal. Pressing Control-v and F3 still prints the correct code for F3, ^[OR.