The function xcb_is_modifier_key, defined in xcb/util-keysyms, file keysyms/keysyms.c, does not recognize ISO_Level5_Shift, ISO_Level5_Latch, and ISO_Level5_Lock as modifiers. This is the same problem (with the same fix) as reported for Xlib in bug 21910, and was probably introduced because the keysyms in keysymdef.h are not listed in ascending order: ISO_Last_Group_Lock appears after ISO_Level5_Lock, but has a smaller value.
A patch is here: http://patchwork.freedesktop.org/patch/6622/
Patch merged.
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