Okay, this is definitely GTK3 bug. I tried to debug GTK3, and found that in gtkimmodule.c it tries to lookup "xim" in some «contexts_hash»
found = g_hash_table_lookup_extended (contexts_hash, *immodules_list, (gpointer *) &context_id, NULL);
It fails, and a bit later in the line
return context_id ? context_id : SIMPLE_ID;
it returns "gtk-im-context-simple".
Okay, this is definitely GTK3 bug. I tried to debug GTK3, and found that in gtkimmodule.c it tries to lookup "xim" in some «contexts_hash»
found = g_hash_ table_lookup_ extended (contexts_hash, *immodules_list,
(gpointer *) &context_id, NULL);
It fails, and a bit later in the line
return context_id ? context_id : SIMPLE_ID;
it returns "gtk-im- context- simple" .