I'm using Plasma 5.18.5 via KDE Neon 5.18 on Ubuntu 18.04. I've been using Compose Key for years, but it stopped working in KDE applications like Kate and Konsole at some point in the last few months. It still works in GTK applications like LibreOffice and SeaMonkey.

I use the Left Win key as my Compose Key. To keep that key from starting the Application Menu, I have the following in ~/.config/kwinrc:

[ModifierOnlyShortcuts]
Meta=

In ~/.config/kxkbrc, I have:

Options=compose:lwin

I created a new user as a test, edited the two files as above, then ran 'kwin_x11 --replace & disown'. Everything works fine for the new user in both KDE apps and GTK apps.

In my normal account, I've tried unsetting Compose Key then setting it back to Left Win, setting it to Right Win, etc., but nothing works. Compose Key continues to work in LibreOffice et al.: 'Left Win' + 3 + 4 gives ¾, but in Kate it gives 34. Any ideas what could be preventing Compose Key from working with KDE apps?