I have a strange problem with my new MacBook Pro running Sierra. After visiting some web sites (such as stack exchange), 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 fixes the F4 key, only quitting Safari does not fix it.
• Disabling Javascript leaves the F4 key enabled.
• Other browsers (Chrome, Firefox) are not affected by this issue.

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