Comment 32 for bug 1573755

ubuntu 16.04 unity: find same problem that .XCompose with alt-right key works in terminal and terminal-based applications (vi, yes I am that old) -- but not in firefox, libreoffice, thunderbird, emacs, etc.

In my case, rm -f ~/.config/dconf/user did not work, nor did any of the other tricks.

Maybe this sheds light on things: my Settings-> Language lists "IBus" as keyboard input method, but ibus does not run on startup/login. Instead, ibus-setup starts with message that demon is not running. And yes, I did follow the proposed fix ("make IBus kbd input method in Languages") -- that did not help. However, after just starting ibus-setup and closing it again, special characters suddenly work in all applications.

However, just starting ibus from terminal as ibus-daemon -d does not work.

There are some ibus-setup messages on the terminal:
/usr/share/ibus/setup/main.py:38: PyGIWarning: Gtk was imported without specifying a version first. Use gi.require_version('Gtk', '3.0') before import to ensure that the right version gets loaded.
  from gi.repository import Gtk
/usr/share/ibus/setup/main.py:39: PyGIWarning: IBus was imported without specifying a version first. Use gi.require_version('IBus', '1.0') before import to ensure that the right version gets loaded.
  from gi.repository import IBus

Can that bring you closer to a solution?