Holding down a key to get accent choices not working despite Terminal command set to "true"

dialabrain's point was that the preference should not exist for normal behavior.

Delete the preference setting and try it out. Don't recreate the thing that is not needed.

If it goes back to not working, then something is interfering with it. Whatever it is might be revealed in an Etrecheck report, but it won't be glaring warning like, "this app is preventing use of ApplePressAndHold." It will just be something you have installed that probably serves no useful purpose, but is somehow related to the keyboard entry.

Early on, there were problems with this feature. One solution was to just add another latin-based language (French, Spanish, etc) to the list of input sources. You don't have to use it, just put it in the list in Keyboard System Prefs > Input Sources. Note that when you add an input source, it will automatically check the box to show it in the menu bar. You can disable that setting so you don't have to see it. I don't know if you even have to keep it in the list. Possibly adding the language and removing could trigger the fix.

How long is “after a while”? If it is a short and predictable period of time you can do two simple tests: 1) use a different account and see if the problem also occurs; 2) boot in Safe Mode and see if the problem occurs.

Of course this not practical if the problem manifests after several hours.

When you detect it has started happening, try logging out and back in. Does it fix it? I am guessing it will.

In order for us to know more and hopefully detect what may be causing it, please run Etrecheck and post its report here.

Try running this command which is supposed to reset the value to the default.

defaults delete NSGlobalDomain ApplePressAndHoldEnabled

These commands may not be valid in High Sierra.

Did you try the read command? When I try it on my iMac Pro I get this…

*****-iMac-Pro:~ *****$ defaults read NSGlobalDomain ApplePressAndHoldEnabled

2018-09-09 07:51:51.103 defaults[56252:7784911]

The domain/default pair of (kCFPreferencesAnyApplication, ApplePressAndHoldEnabled) does not exist

It is perfectly normal to get a “does not exist” reply to the defaults read command. That just mean there is no value set for this property, either because it has not been set or because it was deleted ( with a defaults delete command).

I believe that actually was my situation before I started testing for this thread. But as I said, the setting does work for me.

Well, I remember when it did work because I actually got a "Solved" for providing the Terminal commands. I didn't want to play with the settings since mine works. Thanks for the info on the read command.

"I knew I'd find it"

Keyboard is not repeating when I press and hold the key

I have just tried changing this, and it seems to work on my mac. The setting takes effect after I log out and back in.

FWIW, I always have had this set to false (I actually want keys to repeat).

I set it to true, logged out and back in, and it works as the OP wanted, offering a choice of diacriticals when I press and hold "a", for example.