When typing in some applications on MacOS, pressing and holding a key for example a gives you à á â ä æ ã å ā. That is because the repeating key is enabled. To disable repeating keys in system-wide, you can type:

defaults write -g ApplePressAndHoldEnabled -bool false

In addition, it can be disabled for certain application. For example, disable repeating keys for pycharm, which is very helpful when using plugin IdeaVim.

defaults write com.jetbrains.pycharm ApplePressAndHoldEnabled -bool false

For CLion:

defaults write com.jetbrains.clion ApplePressAndHoldEnable -bool false

For VSCodium:

defaults write com.visualstudio.code.oss ApplePressAndHoldEnabled -bool false

For VSCode:

defaults write com.microsoft.VSCode ApplePressAndHoldEnabled -bool false

Referred from here.