@ericsagnes, thanks for your PR! By analyzing the history of the files in this pull request, we identified @aske, @pSub and @cdepillabout to be potential reviewers.

after cloning 'ericsagnes:pkg-update/fcitx-related' and running sudo nixos-rebuild switch -I /path/to/repo, switching input methods still doesn't work in chromium with the same error:

Apr 26 20:20:47 dn xsession[1520]: [1766:1766:0426/202047.687897:ERROR:browser_main_loop.cc(279)] Gtk: GModule (/nix/store/5a65dlh0pbnyszq7y2q63ykqq84g08qz-fcitx-with-plugins-4.2.9.1/lib/gt
Apr 26 20:20:47 dn xsession[1520]: [1766:1766:0426/202047.687930:ERROR:browser_main_loop.cc(279)] Gtk: Loading IM context type 'fcitx' faile

That's puzzling, does the error also happens with other GTK 2 applications, eg geany?

Could you try the configuration in a clean VM to see if it still happens?
For example you can use the configuration http://nixpaste.nixos.tech-meetups.tokyo/eJX3WYR7 and build a VM with the following command:

$ nixos-rebuild build-vm -I nixpkgs=/path/to/nixpkgs -I nixos-config=/path/to/configuration/file.nix

Also, mozc is somehow touchy with its main server, if you did an update and haven't rebooted afterwards, you can try to reboot that sometimes fix strange state problems.

Installed and tried gedit and got the same sort of error and condition (can't switch input method to mozc)

(gedit:8128): Gtk-WARNING **: GModule (/nix/store/5a65dlh0pbnyszq7y2q63ykqq84g08qz-fcitx-with-plugins-4.2.9.1/lib/gtk-3.0/3.0.0/immodules/im-fcitx.so) initialization check failed: GLib version too old (micro mismatch)
(gedit:8128): Gtk-WARNING **: Loading IM context type 'fcitx' failed

I'll check if I can reproduce this behavior in a vm.

Confirmed that the PR fixes the problem with chromium and other gtk-based applications (I tested gedit and slack)! The reason that nixos-rebuild switch wasn't working was a mistake on my part; I was under the assumption that programs.chromium.enable = true; will install chromium, and had completely forgot about installing it in my root profile.