gtk applications display a composed character ć wrongly

according to the table in https://help.ubuntu.com/community/GtkComposeTable when pressing <compose> <apostrophe> <c> i should get a c with the acute accent, ć. unfortunately, in all gtk applications i get a c with cedilla, ç, which should be obtainable only with <compose> <comma> <c>.

in kde applications the displayed character is correctly the one with acute.

just to see what is going on i ran the keyboard input through the xev and this is the relevant output:

KeyPress event, serial 37, synthetic NO, window 0x5000001,
    root 0xea, subw 0x5000002, time 1493214452, (40,37), root:(1231,763),
    state 0x10, keycode 108 (keysym 0xff20, Multi_key), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: True

KeyRelease event, serial 37, synthetic NO, window 0x5000001,
    root 0xea, subw 0x5000002, time 1493214511, (40,37), root:(1231,763),
    state 0x10, keycode 108 (keysym 0xff20, Multi_key), same_screen YES,
    XLookupString gives 0 bytes:
    XFilterEvent returns: False

KeyPress event, serial 37, synthetic NO, window 0x5000001,
    root 0xea, subw 0x5000002, time 1493217352, (40,37), root:(1231,763),
    state 0x10, keycode 48 (keysym 0x27, apostrophe), same_screen YES,
    XLookupString gives 1 bytes: (27) "'"
    XmbLookupString gives 1 bytes: (27) "'"
    XFilterEvent returns: True

KeyRelease event, serial 37, synthetic NO, window 0x5000001,
    root 0xea, subw 0x5000002, time 1493217418, (40,37), root:(1231,763),
    state 0x10, keycode 48 (keysym 0x27, apostrophe), same_screen YES,
    XLookupString gives 1 bytes: (27) "'"
    XFilterEvent returns: False

KeyPress event, serial 37, synthetic NO, window 0x5000001,
    root 0xea, subw 0x5000002, time 1493217818, (40,37), root:(1231,763),
    state 0x10, keycode 54 (keysym 0x63, c), same_screen YES,
    XLookupString gives 1 bytes: (63) "c"
    XmbLookupString gives 1 bytes: (63) "c"
    XFilterEvent returns: True

KeyPress event, serial 37, synthetic NO, window 0x5000001,
    root 0xea, subw 0x5000002, time 1493217818, (40,37), root:(1231,763),
    state 0x10, keycode 0 (keysym 0x1000107, U0107), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 2 bytes: (c4 87) "ć"
    XFilterEvent returns: False

KeyRelease event, serial 37, synthetic NO, window 0x5000001,
    root 0xea, subw 0x5000002, time 1493217884, (40,37), root:(1231,763),
    state 0x10, keycode 54 (keysym 0x63, c), same_screen YES,
    XLookupString gives 1 bytes: (63) "c"
    XFilterEvent returns: False

as you can see, the xev correctly displays the character in the terminal, but then typing it in the same terminal (without xev) directly into bash gives the one with cedilla. repeating the same exercise in kosole gives the correct result, so imho this must be related to gnome/gtk.

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: libgnomekbd-common 3.22.0.1-1
ProcVersionSignature: Ubuntu 4.10.0-37.41-generic 4.10.17
Uname: Linux 4.10.0-37-generic x86_64
ApportVersion: 2.20.4-0ubuntu4.5
Architecture: amd64
CurrentDesktop: XFCE
Date: Mon Oct 30 15:52:18 2017
InstallationDate: Installed on 2014-10-21 (1105 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
PackageArchitecture: all
SourcePackage: libgnomekbd
UpgradeStatus: Upgraded to zesty on 2017-08-31 (60 days ago)