integration: add input method section

src/integration.md

@@ -14,6 +14,7 @@
 7. [Session Lock](#session-lock)
 8. [Desktops](#desktops)
 9. [Qt](#qt)
+10. [Input Method](#input-method)
 
 # 1. Introduction {#introduction}
 
@@ -328,6 +329,84 @@ and set `QT_QPA_PLATFORMTHEME=qt6ct` in `~/.config/labwc/environment`.
 
 [qt6-wayland]: https://archlinux.org/packages/extra/x86_64/qt6-wayland/
 [qt6ct]: https://github.com/trialuser02/qt6ct
+
+# 10. Input Method {#input-method}
+
+Input methods like Fcitx5 and IBus provide modules for GTK and Qt and an
+interface for xserver (xwayland) using D-Bus without wayland protocols.
+
+You can enable input method for those platforms by setting environment
+variables like this:
+
+```
+export GTK_IM_MODULE=fcitx
+export QT_IM_MODULE=fcitx
+export XMODIFIERS=@im=fcitx
+```
+
+<!-- TODO: remove "in the master branch" when labwc 0.7.2 is released -->
+For apps not running on those platforms (e.g. Alacritty), labwc supports
+the following wayland protocols in the master branch:
+
+- [text-input-v3]
+  - Used by winit apps (e.g. Alacritty) and GTK (without setting
+    `GTK_IM_MODULE` environment variable).
+- [input-method-v2]
+  - Supported by Fcitx5, but not by IBus yet ([issue]).
+
+Here is a quick guide for using Fcitx5 in labwc:
+
+1. Install fcitx5, GTK/Qt modules, configtool and language-specific module.
+    - Arch Linux: `pacman -S fcitx5-im fcitx5-mozc`
+    - Ubuntu: `apt install fcitx5 fcitx5-mozc`
+
+    Replace `fcitx5-mozc` with a module for your language.
+
+2. Set `GTK_IM_MODULE`, `QT_IM_MODULE` and `XMODIFIERS` like described earlier.
+These are usually saved in files like `~/.profile`, `~/.bash_profile` and
+`/etc/environment`.
+
+3. Start `fcitx5`. You can automatically start fcitx5 by adding `fcitx5 &` to
+`~/.config/labwc/autostart`.
+
+4. Configure Fcitx5 with configtool to enable the installed language-specific
+module and to set up hotkeys. See [Configtool (Fcitx 5) - Fcitx].
+
+5. Activate input method with hotkeys while typing in applications.
+
+<!--- TODO: remove this once https://chromium-review.googlesource.com/c/chromium/src/+/3750452 is merged -->
+## Input method on Chromium
+
+Chromium (and Electron-based apps) currently doesn't support [text-input-v3]
+protocol. So if you want to use IME with Chromium under labwc, you have
+following options:
+
+1. Run Chromium under XWayland
+
+    This is the default option. However, some features like touchpad gesture
+    don't work.
+
+2. Use GTK IM Module
+
+    By running Chromium with `--enable-features=UseOzonePlatform --ozone-platform=wayland --gtk-version=4`,
+    you can enable IME with GTK IM Module (selected by `GTK_IM_MODULE`) under
+    wayland. However, IME popups might be incorrectly positioned.
+
+3. Patch labwc and use [text-input-v1] protocol
+
+    Since [text-input-v1] is an outdated protocol, labwc doesn't officially
+    support it. However, you can optionally add support for it by installing
+    labwc from the unofficial [AUR] or by applying its patch. Then, you can
+    enable IME with [text-input-v1] by running Chromium with
+    `--enable-features=UseOzonePlatform --ozone-platform=wayland --enable-wayland-ime`.
+
+[text-input-v3]: https://wayland.app/protocols/text-input-unstable-v3
+[input-method-v2]: https://wayland.app/protocols/input-method-unstable-v2
+[issue]: https://github.com/ibus/ibus/issues/2182
+[Configtool (Fcitx 5) - Fcitx]: https://fcitx-im.org/wiki/Configtool_(Fcitx_5)
+[text-input-v1]: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/blob/main/unstable/text-input/text-input-unstable-v1.xml
+[AUR]: https://aur.archlinux.org/packages/labwc-im
+
 [waybar repository]: https://github.com/Alexays/Waybar
 [waybar documentation]: https://github.com/Alexays/Waybar/tree/master/man
 [xfce4-panel repository]: https://gitlab.xfce.org/xfce/xfce4-panel