Comment 6 for bug 919899

I guess I may have found a workaround.
I use Hungarian keyboard layout with hu_HU.UTF-8 locale and I'd like to type the "ć" character with AltGr+9+c (dead_acute+c) (used in Polish and Croatian/Serbian).
I have been googling and experimenting quite long to find a workaround with no success. The most interesting point was, that the file that most claimed needs the changes already contains the proper entries:

/usr/share/X11/locale/en_US.UTF-8/Compose

<dead_acute> <C> : "Ć" U0106 # LATIN CAPITAL LETTER C WITH ACUTE
<Multi_key> <acute> <C> : "Ć" U0106 # LATIN CAPITAL LETTER C WITH ACUTE
<Multi_key> <apostrophe> <C> : "Ć" U0106 # LATIN CAPITAL LETTER C WITH ACUTE
<Multi_key> <C> <apostrophe> : "Ć" U0106 # LATIN CAPITAL LETTER C WITH ACUTE
<dead_acute> <c> : "ć" U0107 # LATIN SMALL LETTER C WITH ACUTE
<Multi_key> <acute> <c> : "ć" U0107 # LATIN SMALL LETTER C WITH ACUTE
<Multi_key> <apostrophe> <c> : "ć" U0107 # LATIN SMALL LETTER C WITH ACUTE
<Multi_key> <c> <apostrophe> : "ć" U0107 # LATIN SMALL LETTER C WITH ACUTE

I tried changing other files as well:

- /etc/console-setup/compose.ISO-8859-1.inc (I changed the lines combining dead_acute with c to result in "ć" instead of "ç", but it did not help)
- adding a ~/.XCompose with the proper entries (but what for, if en_US.UTF-8/Compose already contains the proper entries?)

As a last act of desperation, basing on a hint I found at http://wiki.linuxquestions.org/wiki/Accented_Characters I tried adding "GTK_IM_MODULE=xim" to /etc/environment. This finally helped.
Then I reverted all my previous changes (left just the one in /etc/environment), and entering "ć" is still possible.
However now I see other problems with the input method (Ctrl+F doesn't work in gedit - when the IM_MODULE is not xim it works properly), so I wouldn't like to use this "hack".

So my questions are:
- Why are the settings in /usr/share/X11/locale/en_US.UTF-8/Compose not taking any effect with the default input method? (my /usr/share/X11/locale/compose.dir defines en_US.UTF-8/Compose for hu_HU.UTF-8, so it should take effect)
- Why are those settings taking effect in turn when the input method is xim?
- What is the default input method (I guess it's not xim), and where are those "Compose" settings that should take effect for it?