[Bug][Linux] AppImage: IBus input methods don't work — bundled immodules.cache has no im-ibus, GTK falls back to gtk-im-context-simple

[pfoot]

Describe the bug

In the Linux AppImage, CJK input through IBus doesn't reach any text field. With ibus-hangul, English typing works, but in Hangul mode keystrokes produce nothing and the Hangul/English toggle has no effect. Other GTK apps in the same session work normally.

Environment

• Buzz desktop 0.5.23, Buzz_0.5.23_amd64.AppImage from GitHub releases
• Ubuntu 24.04, GNOME on X11, NVIDIA
• IBus 1.5.29 + ibus-hangul (ibus-daemon --panel disable --xim, started by GNOME)
• LANG=en_US.UTF-8, XSettings gtk-im-module = ibus

Cause

apprun-hooks/linuxdeploy-plugin-gtk.sh unconditionally exports

export GTK_IM_MODULE_FILE="$APPDIR//usr/lib/x86_64-linux-gnu/gtk-3.0/3.0.0/immodules.cache"

The bundled cache lists only GTK's built-in modules (am-et, broadway, cedilla, cyrillic-translit, inuktitut, ipa, multipress, thai, ti-er, ti-et, viqr, wayland, xim) — no im-ibus.so, and no fcitx module either. GTK asks for the ibus module named by XSettings, doesn't find it in the cache, and silently falls back to gtk-im-context-simple, which never talks to IBus.

Reproduced outside Buzz with a minimal GTK 3 script (create a GtkIMMulticontext, focus_in(), print get_context_id()):

GTK_IM_MODULE_FILE	chosen IM context
host default	ibus
Buzz's bundled cache	gtk-im-context-simple

Workaround

Force the bundled XIM module, which talks to IBus's XIM server:

GTK_IM_MODULE=xim \
LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/gtk-3.0/3.0.0/immodules \
./Buzz_0.5.23_amd64.AppImage

(LD_LIBRARY_PATH was needed in my host-GTK test because the cache names modules by bare filename; it may be unnecessary with the bundled GTK.)

Hangul input then works, but only off-the-spot: each syllable is composed in a popup outside the field and inserted when it commits. That is usable but noticeably worse than inline preedit in native GTK apps.

Suggested fix

Similar in spirit to #6971 (pointing the AppImage at host GStreamer plugins): let the bundled GTK see the host's IM modules instead of shadowing them, e.g. in desktop/scripts/fix-appimage.sh:

• don't override GTK_IM_MODULE_FILE when the host has a cache (/usr/lib/x86_64-linux-gnu/gtk-3.0/3.0.0/immodules.cache, /usr/lib64/gtk-3.0/3.0.0/immodules.cache, …), or
• generate a merged cache (host entries + bundled) at startup under $XDG_RUNTIME_DIR, or
• bundle im-ibus.so and the fcitx5 GTK 3 module.

origin/main (77729ab) has no IM handling in fix-appimage.sh, so this likely still reproduces there.

Possibly related

• Keyboard input freezes app when running under XWayland with input method enabled #4854 (keyboard freeze under XWayland with an input method) — different symptom, same AppImage IM path.
• During onboarding, the username field accepted only one typed character; pasting the whole name worked. Not re-tested after the workaround.