evkoverlay is a keyboard overlay utility that runs on Linux and particularly Wayland because it grabs input directly from Linux's evdev interface. This unfortunately means that it has to be run as root in order to be able to access files in /dev/input/, but I really cannot think of any way around that for now.
This tool is written in Haskell and is built using
stack, just run stack install to build and install the executable.
Raylib is needed.
Before usage, you must change the value of configInputFile in
app/Main.hs (and of course rebuild) to whatever your keyboard
device is. To find out, look in /proc/input/by-path and look for
any file that vaguely looks like a keyboard (kbd, keyboard, etc.),
for example /dev/input/by-path/platform-i8042-serio-0-event-kbd.
This only needs to be done once.
As mentioned, this tool must be run with elevated privileges (sudo evkoverlay) in order to access raw input devices. Configuration,
e.g. changing your keys or layout, can be done by editing
app/Main.hs and rebuilding. The values of configSizeofTimeT = 8 and
configSizeofSusecondsT = 8 should be correct for most people, but if
you are on an exceptionally odd system they may require changing.