Comment 11 for bug 1514104

Ok so i managed to gather enough info,
https://github.com/p-e-w/finalterm/issues/364
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=690270

This is a known bug, it won't be corrected using xim input method is not working:
https://bugs.launchpad.net/synapse-project/+bug/1219314

The solution is to:

Replace in /usr/local/share/applications/synapse.desktop
Exec=synapse
by
Exec=GTK_IM_MODULE='' synapse

Use an alias for synapse if launching form CLI
Use another input method with sudo im-config

I think i would be really nice to update wiki or something.