In macOS v10.12 (Sierra), if I hold down the key, a menu of blue letters with accents and diacritical marks appears.
The prior behavior was to enter a string of the letters pressed liiiiiiiike thiiiiiis. How do I disable this menu, so I can type many of the same letter?
Open a Terminal window and run defaults write -g ApplePressAndHoldEnabled -bool false. Some programs will reflect the new setting immediately, but you may need to restart programs like Slack in order to get them to load the setting.
This answer is from Disable the character accent menu and enable key repeat in Mac OS X.
Related Super User content is at after holding a key for a while I can't type until i close and re-open the app.