macOS: Disable popup showing accented characters when holding down a key
In macOS, when a key is held down while entering text, a popup is shown which lets one choose between various accented forms of the character. To disable this execute the following command-line in the Terminal.app:

defaults write -g ApplePressAndHoldEnabled -bool false

Now, you'll need to log-out and log back in. This should disable the display of the popup and character typed should start repeating when the key is held down.

If you ever wish to return to this behaviour, execute the following command-line in the Terminal.app:

defaults write -g ApplePressAndHoldEnabled -bool true

You'll need to log-out and log back in again for the setting to take effect.