Additional characters when I press longer How to turn those additional characters off? After updating to macOS Sierra when I hit a key a little bit longer (it's the same amount of time I always did) i get this and it's constantly disrupting my flow
This "feature" has been around for a few years.  See if this helps:

https://www.macobserver.com/tmo/article/os_x_lion_using_and_disabling_the_character_picker