I'm running macOS High Sierra v10.13.3.
Holding a key — typically a vowel — down, by default, will enable a pop-up of diacritics to appear. A while back, I had apparently disabled this function, thereby enabling key repeat.
I had wanted to reenable this default, and therefore disable key repeat. The most obvious method that I had found via numerous forums and blog posts was entering the following into Terminal:
defaults write -g ApplePressAndHoldEnabled -bool true
I've performed this operation, and then restarted my computer, about a dozen times, and I still cannot disable key repeat and enable the accent mark pop-up to appear. As an additional remedy, I disabled key repeat in System Preferences — but to no avail.
Funny thing is: the feature works exclusively on TextEdit and no other application.