Comment 7 for bug 1836659

$ echo $GTK_IM_MODULE
ibus

I don't know which session I use, I set this up a loong time ago and I have no idea how to find out.
But yes, it looks like it's flashback:

$ ps -ef|grep -i flash
hajma 1742 1503 0 čec16 ? 00:00:00 /bin/sh /usr/lib/gnome-session/run-systemd-session gnome-session-flashback.target
hajma 1910 1742 0 čec16 ? 00:00:00 /usr/bin/ssh-agent /usr/bin/im-launch /usr/lib/gnome-session/run-systemd-session gnome-session-flashback.target
hajma 2017 1742 0 čec16 ? 00:00:00 systemctl --user start --wait gnome-session-flashback.target
hajma 2052 1721 0 čec16 ? 00:00:00 /usr/lib/gnome-session/gnome-session-binary --session=gnome-flashback-metacity --disable-acceleration-check
hajma 2309 2052 0 čec16 ? 00:00:41 gnome-flashback
hajma 3343 3228 0 16:59 pts/0 00:00:00 grep --color=auto -i flash