Dead keys suddenly stopped working in GTK applications

I use several keyboards layouts on a regular basis. Two of which make use of dead keys for French language. $ cat /etc/default/keyboard XKBLAYOUT=us,fr,fr XKBVARIANT=dvp,bepo,latin9 BACKSPACE=guess This morning, after a sudo apt-get upgrade on Debian 9 (stretch), which may or may not be involved in the problem, dead keys stopped working as dead keys (*i.e.* hitting ^ then e produces ^e instead of ê), which makes typing french quite difficult. The new behaviour is persistent after reboot and affects several applications: - firefox - thunderbird - gnome-terminal - gedit - nautilus But **not** VLC or xterm.. what should I conclude? How do I get my dead keys working again in all applications?