Compose key stopped working in KDE for GTK applications, Fedora 43

Compose key, which is used for typing uncommon characters, no longer works in GTK4 applications in KDE, on Fedora 43.

This affects regular applications and Flatpak ones.

TL;DR: use GTK_IM_MODULE=simple environment variable.

Some time ago I spotted that XCompose bindings stopped working in Gnome applications.

Apparently, starting with GTK4 4.19.2, GTK4 uses Wayland's text input protocol by default.
It is supported in KDE, but for yet undebugged reasons stopped working.

https://github.com/GNOME/gtk/blob/8784688d974be9a9049382b7295b2fe57108061b/NEWS#L1523-L1526

Overview of Changes in 4.19.2, 27-06-2025

Note: This release makes GTK rely on the Wayland text protocol for input method support. For compositors without that protocol, you can manually set GTK_IM_MODULE=simple to get back support for X11-style compose sequences and dead keys.

Running applications with GTK_IM_MODULE=simple indeed continue to work, which is a workaround for now.