Control: notfound -1 0.30-1 This appears to be my mistake. After checking im-config script for ibus (/usr/share/im-config/data/21_ibus.rc), im-config already attempts to use ibus GTK IM Module when available, and falls back to XIM otherwise. My system just happenned to have libqt5gui5 installed, and the ibus package alternative dependencies were just satisfied without ibus-gtk. When I install ibus-gtk manually, GTK_IM_MODULE is properly set to "ibus" as desired.
So, I'm closing this bug as notfound. Sorry for the noise. Regards, -- Theppitak Karoonboonyanan http://linux.thai.net/~thep/