How can I enable this? You know, when you press-and-hold a key, a small palette will appear offering alternative characters similar to the one you’re holding down. I've set QT_IM_MODULE=plasmaim, but to no avail.

Got it working now. Great feature if you have a lot of locale accents and hate a compose key (like me). Used Kcharselect which is great, but this feature increases workflow quite a bit. Couldn't get it to work prior to 5.24 for some reason. Still use Kchar for the really special signs though.https://imgur.com/a/vlhiXPA