When running a wayland session, left handed mouse mode works for wayland windows. But running e.g. Firefox in a regular xwayland window, the mouse buttons are inverted (left/right click). Clicking the physical right button should trigger a "left click" but results in a "right click".
Actually it seems like qt 4 applications are not effected, but visual studio code and firefox get the mouse buttons inverted.
Sorry not much KWin can do about that. KWin does not do anything special for left handed. It just informs libinput to use left handed mode and that reports buttons. KWin just forwards them. If they work in some areas but not all, it cannot be in KWin. It looks to me like a bug in XWayland.
The problem described here affects me on Arch Linux running plasma 5.12.5 and xwayland 1.19.6+13+gd0d1a694f-2. QT4 app (Clementine music player) is also affected. My laptop also runs Arch Linux and obviously the same xwayland version but with Gnome 3.28.1 and I can not reproduce such problem under its Wayland session. What do you think Martin?
I have no idea. I could imagine gnome doing workarounds. Anyway this needs to be reported to xwayland.
Reported https://bugs.freedesktop.org/show_bug.cgi?id=106424