macos Sierra Fn key acts like a long press with delay I have recently updated to macos Sierra and have a very strange thing happening with my Fn key. I have looked everywhere but cannot find anyone with the same issue. 

Whenever I press the Fn key, it looks like the first key I press immediately after holding it is not recorded, instead, it acts like a long press of that key, and only the second press acts as normal.

For example: I do Fn+Delete to forward-delete, then it deletes as normal delete, and then starts deleting everything after a second, exactly like if I was just long pressing the delete key. If I press the delete key a second time (while holding the Fn), then it does act as expected. The funny thing is that once I release the Fn key, whatever I press after having released it still acts like if I was still pressing Fn, and then the rest of the key presses are normal again.

I first saw this issue with Fn+Delete but it happens with all keys, Fn+RightArrow will make my cursor go through all the text, etc. I have tried a SMC reset and a PRAM reset, I logged out and logged in to a newly created account to see if it was some issue in my user account, but nothing seems to solve it. I also tried turning sticky keys on and off. 

Please help, I can't think of anything else.
Are you using USB Overdrive?  I went into the USB Overdrive settings, selected "Any keyboard, any application" and deselected the Enabled checkbox, and the problem seems to have disappeared